The effects of partial and total interosseous membrane transection on load sharing in the cadaver forearm.

This study was performed to examine the effects of partial and total transection of the interosseous membrane (IOM) on load transfer in the forearm. Twenty fresh frozen forearms were instrumented with custom designed load cells placed in the proximal radius and distal ulna. Simultaneous measurements of load cell forces, radial head displacement relative to the capitellum, and local tension within the central band of the IOM were made as the wrist was loaded to 134 N with the forearm at 90 degrees of elbow flexion and in neutral pronation supination. For valgus elbow alignment (radial head contacting the capitellum), mean force carried by the distal ulna was 7.1% of the applied wrist force and mean force transferred from radius to ulna through the IOM was 4.4%. For varus elbow alignment (mean 2.0 mm gap between the radial head and capitellum), mean distal ulna force was 28% and mean IOM force was 51%. Section of the proximal and distal one-thirds of the IOM had no significant effect upon mean distal ulnar force or mean IOM force. Total IOM section significantly increased mean distal ulnar force for varus elbow alignment in all wrist positions tested. The mean level of applied wrist force necessary to close the varus gap (89 N) decreased significantly after both partial IOM section (71 N) and total IOM section (25 N). The IOM became loaded only when the radius displaced proximally relative to the ulna, closing the gap between the radius and capitellum. As the radius displaced proximally, the wrist becomes increasingly ulnar positive, which in turn leads to direct loading of the distal ulna. This shift of force to the distal ulna could present clinically as ulnar sided wrist pain or as ulnar impaction after IOM injury.

Effects of radial head excision and distal radial shortening on load-sharing in cadaver forearms.

BACKGROUND: The present study was performed to measure changes in radioulnar load-sharing in the cadaveric forearm following two orthopaedic surgical procedures that often have varying results: radial head excision and distal radial shortening. A better understanding of the biomechanical consequences of those procedures could aid surgeons in obtaining a more satisfactory clinical outcome. METHODS: Miniature load-cells were inserted into the proximal part of the radius and the distal part of the ulna in twenty fresh-frozen cadaveric forearms. Load-cell forces, radial head displacement relative to the capitellum, and local tension within the central band of the interosseous membrane were measured simultaneously as the wrist was loaded to 133.5 N in neutral pronation-supination and neutral radioulnar deviation. Testing was repeated after incremental distal radial shortening and after removal of the radial head. RESULTS: With the elbow flexed to 90 degrees and in valgus alignment (the radial head in contact with the capitellum), the mean force in the distal part of the ulna was 7.1% of the applied wrist force and the mean force in the interosseous membrane was 4.0%. With the elbow in varus alignment (a mean initial gap of 1.97 mm between the radial head and the capitellum), the respective mean values were 27.9% and 51.2%. After excision of the radial head, the mean force in the distal part of the ulna increased to 42.4% of the applied wrist force and the mean force in the interosseous membrane increased to 58.8%, in both varus and valgus elbow alignment. The mean distal ulnar force increased with progressive distal radial shortening in both varus and valgus elbow alignment; after 6 mm of radial shortening, the distal ulnar force averaged 92.4% (in varus alignment) and 60.9% (in valgus alignment). Equal distal load-sharing between the radius and ulna occurred after approximately 5 mm of radial shortening with the elbow in valgus alignment and after approximately 2 mm of radial shortening with the elbow in varus alignment. In valgus alignment, the force in the interosseous membrane was negligible after all degrees of radial shortening; in varus alignment, the mean force in the interosseous membrane decreased from 51.2% (0 mm of distal radial shortening) to 0% (6 mm of distal radial shortening) because of progressive slackening of the interosseous membrane. CONCLUSIONS: Radial head excision shifted the applied wrist force that normally would be transmitted to the elbow, through radial head-capitellar contact, to the interosseous membrane. The resulting proximal radial displacement created an ulnar-positive wrist and increased distal ulnar loading. Radial shortening and ulnar lengthening procedures have been designed to shift the applied wrist force from the distal part of the radius to the distal part of the ulna; it is commonly assumed that these procedures have equivalent biomechanical effects. We found that radial shortening resulted in slackening of the interosseous membrane, thereby negating its ability to transmit load through the forearm. Slackening of the interosseous membrane would not be expected with distal ulnar lengthening procedures. CLINICAL RELEVANCE: When the radial head has been fractured or excised, the mechanical status of the interosseous membrane is critical to the load-sharing process. If the interosseous membrane remains intact, distal ulnar loads will be limited to less than half of the applied wrist force; if the interosseous membrane has been damaged, nearly the entire applied wrist force will be shifted to the ulna. The amount of radial shortening or ulnar lengthening performed at the time of surgery during joint-leveling procedures has been largely empirical. We found that distal ulnar load increased by approximately 10% for each millimeter of radial shortening.

Mechanisms of load transfer in the cadaver forearm: role of the interosseous membrane.

Forces transmitted through the distal ulna and proximal radius, relative motion between the radial head and capitellum, and measurements of tissue strain and local fiber tension within the central band of the interosseous membrane were recorded as cadaveric forearms were loaded axially through the wrist. With the elbow in valgus alignment (the radial head in direct contact with the capitellum), an average of 93% of force applied to the wrist was transferred directly through the radius to the elbow with no appreciable load transfer through the interosseous membrane. With varus alignment (initial gap between the radial head and capitellum) load applied to the wrist displaced the radius proximally an average of 1.1 mm until radial head contact occurred at a mean applied wrist force of 89.0 N. Proximal displacement of the radius generated strain in the central band of the interosseous membrane and created a more ulnar positive wrist, which in turn increased distal ulnar loading; distal ulnar force averaged 19% and interosseous membrane averaged 54% of applied wrist force. Distal ulnar loading was unaffected by 25 degrees wrist flexion-extension or by 20 degrees of radioulnar deviation. With 40 degrees ulnar deviation, mean distal ulnar forces were 18% and 48% of applied wrist force for valgus and varus elbow alignments, respectively. Mean load-sharing percentages at the wrist and elbow were not significantly different between 222. 5 N and 133.5 N of applied force for any wrist position and were unaffected by the angle of elbow flexion.

In situ calibration of miniature sensors implanted into the anterior cruciate ligament part I: strain measurements.

The goals of this study were to (a) evaluate the differential variable reluctance transducer as an instrument for measuring tissue strain in the anteromedial band of the anterior cruciate ligament, (b) develop a series of calibration curves (for simple states of knee loading) from which resultant force in the ligament could be estimated from measured strain levels in the anteromedial band of the ligament, and (c) study the effects of knee flexion angle and mode of applied loading on output from the transducer. Thirteen fresh-frozen cadaveric knee specimens underwent mechanical isolation of a bone cap containing the tibial insertion of the anterior cruciate ligament and attachment of a load cell to measure resultant force in the ligament. The transducer (with barbed prongs) was inserted into the anteromedial band of the anterior cruciate ligament to record local elongation of the instrumented fibers as resultant force was generated in the ligament. A series of calibration curves (anteromedial bundle strain versus resultant force in the anterior cruciate ligament) were determined at selected knee flexion angles as external loads were applied to the knee. During passive knee extension, strain readings did not always follow the pattern of resultant force in the ligament; erratic strain readings were often measured beyond 20 degrees of flexion, where the anteromedial band was slack. For anterior tibial loading, the anteromedial band was a more active contributor to resultant ligament force beyond 45 degrees of flexion and was less active near full extension; mean resultant forces in the range of 150-200 N produced strain levels on the order of 3-4%. The anteromedial band was also active during application of internal tibial torque; mean resultant forces on the order of 180-220 N produced strains on the order of 2%. Resultant forces generated by varus moment were relatively low, and the anteromedial band was not always strained. Mean coefficients of variation for resultant force in the ligament (five repeated measurements) ranged between 0.038 and 0.111. Mean coefficients of variation for five repeated placements of the strain transducer in the same site ranged from 0.209 to 0.342. Insertion and removal of this transducer at the anteromedial band produced observable damage to the ligament. In our study, repeatable measurements were possible only if both prongs of the transducer were sutured to the ligament fibers.

In situ calibration of miniature sensors implanted into the anterior cruciate ligament part II: force probe measurements.

The arthroscopically implantable force probe transducer, which measures the effects of local ligament fiber tension, was inserted into the anteromedial band of the anterior cruciate ligament after measurements with the differential variable reluctance transducer were completed in Part I of this study. The overall goals in Part II remained the same, with additional experiments included to determine the sensitivity of output voltage from the transducer to medial-lateral placement of the device within the anteromedial band and to depth of placement within a given insertion hole. Calibration curves of output voltage from the arthroscopically implantable force probe transducer versus resultant force in the ligament were generated during a separate series of knee-loading experiments identical to those performed in Part I. The output voltage for a given probe placement was highly sensitive to the depth of implantation into the anteromedial band. When the probe was completely buried within the ligament, voltage outputs were often sporadic or absent even though surface fibers had clearly developed tension. When the probe was only partially inserted into the hole, such that the end of the probe was slightly proud to the surface, voltage output was significantly higher as the device measured tension in the superficial fibers. Voltage outputs for proud placement were always significantly higher than corresponding voltages for deep placements for all test conditions. With proud placements, voltage outputs were not sensitive to small deviations in medial-lateral position within the anteromedial band. Mean coefficients of variation for output voltage (four repeated placements of the probe into the same central hole) ranged from 0.156 to 0.359 (deep and proud insertions). Output voltage from the probe generally followed the pattern of resultant force in the ligament during passive knee extension. For anterior tibial loading, the contribution of deep fibers to resultant force did not depend on the knee flexion angle at which the test was conducted; the contribution of superficial fibers was greatest beyond 45 degrees of flexion and least at full extension. The contributions of the anteromedial band to resultant force in the ligament were not significantly different between the three modes of loading (anterior tibial force, internal tibia torque, and varus moment) at either 0 or 10 degrees of flexion; this was true for both superficial and deep fibers. We found it necessary to secure the probe within the insertion site using a suture (for both deep and proud placements) to obtain repeatable readings. Puncturing the anteromedial band clearly produced tissue damage; the insertion hole often produced a permanent plane of cleavage in the anteromedial band. However, this tissue damage did not alter the overall ability of the ligament to generate resultant force.

Radioulnar load-sharing in the forearm. A study in cadavera.

Custom-designed miniature load-cells were inserted into the distal end of the ulna and the proximal end of the radius in ten fresh-frozen forearms from cadavera. The forces transmitted through the bones at these sites were measured under 134 newtons of constant axial load that was applied through the metacarpals as the forearm was rotated from 60 degrees of supination to 60 degrees of pronation. The simultaneous measurements of these forces allowed the calculation of radioulnar load-sharing at the wrist and the elbow as well as the calculation of the amount of force that was transferred from the radius to the ulna through the interosseous membrane. With the elbow in valgus alignment (that is, with contact between the radial head and the capitellum), the main pathway for load transmission through the forearm was direct axial loading of the radius; measurements from both load-cells were unaffected by the angle of elbow flexion. When the forearm was in neutral rotation, the mean force in the distal end of the ulna averaged 2.8 per cent of the load applied to the wrist and the mean force in the proximal end of the ulna averaged 11.8 per cent; this indicated that only a small amount of tension developed in the interosseous membrane. With the elbow in varus alignment (that is, with no contact between the radial head and the capitellum), load was transmitted through the forearm by a transfer of force from the radius to the ulna through the interosseous membrane. When the forearm was in neutral rotation, the force in the distal end of the ulna averaged 7.0 per cent of the load applied to the wrist and the force in the proximal end of the ulna averaged 93.0 per cent; the force through the interosseous membrane decreased with supination of the forearm. Testing with the elbow in valgus alignment and shortening of the distal end of the radius in two-millimeter increments produced corresponding increases in force in the distal end of the ulna and decreases in force in the radial head. The forces through the interosseous membrane remained low after each amount of radial shortening.

A biomechanical study of replacement of the posterior cruciate ligament with a graft. Part 1: Isometry, pre-tension of the graft, and anterior-posterior laxity.

UNLABELLED: Twelve fresh-frozen knee specimens from cadavera were subjected to anterior-posterior laxity testing with 200 newtons of force applied to the tibia; testing was performed before and after a femoral load-cell was connected to a mechanically isolated cylindrical cap of subchondral femoral bone containing the femoral origin of the posterior cruciate ligament. The posterior cruciate ligament then was removed, the proximal end of a thin trial isometer wire was attached to one of four points designated on the femur, and displacement of the distal end of the wire relative to the tibia was measured over a 120-degree range of motion. The potted end of a ten-millimeter-wide bone-patellar ligament-bone graft was centered over the femoral origin of the ligament and attached to the femoral load-cell. Isometry measurements were repeated with the wire attached to the bone block of the free end of the graft in the tibial tunnel. Force was recorded at the load-cell (representing force in the intra-articular portion of the graft) as pre-tension was applied, with use of a calibrated spring-scale, to the tibial end of the graft. A laxity-matched pre-tension of the graft was determined such that the anterior-posterior laxity of the reconstructed knee at 90 degrees of flexion was within one millimeter of the laxity that was measured after installation of the load-cell. Anterior-posterior testing was repeated after insertion of the graft at the laxity-matched pre-tension. The least amount of change in the relative displacement of the trial wire over the 120-degree range of flexion occurred when the wire was attached to the proximal point on the femur (a point on the proximal margin of the femoral origin of the posterior cruciate ligament, midway between the anterior and posterior borders of the ligament). The greatest change in the relative displacement was associated with the anterior point (a point on the anterior margin of the femoral origin of the ligament, midway between the proximal and distal borders). The mean relative displacements of the trial wire when it was attached to a point at the center of the femoral origin of the ligament were not significantly different from the corresponding mean displacements of the distal end of the graft when the proximal end of the graft was centered at this point. At 90 degrees of flexion, the force recorded by the load-cell averaged 64 to 74 per cent of the force applied to the tibial end of the graft. The laxity-matched pre-tension of the graft at 90 degrees of flexion (as recorded by the load-cell) ranged from six to 100 newtons (mean and standard deviation, 43.0 +/- 33.4 newtons). With the numbers available, the mean laxities after insertion of the graft were not significantly different, at any angle of flexion, from the corresponding mean values after installation of the load-cell. CLINICAL RELEVANCE: Isometer readings from a trial wire attached to a point on the femur provided an accurate indication of the change in the length of a graft subsequently centered at that point. Anteriorly placed femoral tunnels should be avoided, as the isometer readings indicated increased tension, with flexion of the knee, in a graft placed in this region. The force in the intra-articular portion of the graft was always less than the force applied to the bone block in the tibial tunnel. Therefore, the femoral end of the graft should be tensioned to avoid frictional losses from the severe bend in the graft as it passes over the posterior tibial plateau. With correct pre-tensioning of a graft, normal anterior-posterior laxity at 0 to 90 degrees of flexion can be restored. However, because of the considerable range in the laxity-matched pre-tensions, we recommend that the pre-tension be greater than forty-three newtons for all patients to ensure that normal laxity is restored.

A biomechanical study of replacement of the posterior cruciate ligament with a graft. Part II: Forces in the graft compared with forces in the intact ligament.

UNLABELLED: A femoral load-cell was installed in twelve fresh-frozen knee specimens from cadavera, to measure the resultant force at the femoral origin of the posterior cruciate ligament during a series of tibial-loading tests. The posterior cruciate ligament was removed, and a ten-millimeter-wide bone-patellar ligament-bone graft was inserted. The knee was flexed to 90 degrees, the graft was pre-tensioned to restore the anterior-posterior laxity to that recorded after installation of the load-cell, and the loading tests were repeated. With the tibia locked in neutral rotation and a 200-newton posterior force applied to the tibia, the mean force generated in the intact posterior cruciate ligament ranged from 220 newtons at 90 degrees of flexion to thirty-six newtons at full extension. When the tibia was locked in external rotation during the posterior drawer test, the force was reduced when the knee was flexed 10 to 70 degrees; when the tibia was locked in internal rotation, the mean force was reduced at only 30 and 45 degrees of flexion. The mean forces in the graft were not significantly different, with the numbers available, from the corresponding values for the intact ligament during application of a straight posterior tibial force (neutral tibial rotation), during application of a fifteen-newton-meter flexion or extension moment (hyperflexion or hyperextension), during application of a ten-newton-meter varus or valgus moment, or during application of a ten-newton-meter internal or external tibial torque. With the numbers available, there were no significant differences between the mean tibial rotations associated with the intact posterior cruciate ligament and those associated with the graft at any angle of flexion, without or with applied tibial torque. CLINICAL RELEVANCE: The amount of force generated in the posterior cruciate ligament during the posterior drawer test depends on the angle of flexion at which the test is performed. When the angle of flexion is near 90 degrees, all of the posterior force applied to the tibia is transmitted to the ligament and the force in the ligament is not affected by the position of tibial rotation. When the test is performed at an angle of flexion near 30 degrees and in neutral tibial rotation, other structures (such as the collateral ligaments and the posterior part of the capsule) help to resist the posterior force applied to the tibia. The position of tibial rotation is important when the test is performed with the knee at an angle of flexion near 30 degrees, as secondary structures pre-tensioned by tibial torque act to reduce the amount of force carried by the posterior cruciate ligament even more. With a few minor exceptions, we found that the forces in a graft used to replace the posterior cruciate ligament were approximately the same as those in the intact ligament. Therefore, there appears to be little justification for restricting low-level rehabilitation activities once the fixation of the graft has healed. However, forces in the graft could be quite high during hyperextension and hypertension, as they are in the intact ligament. Thus, bracing in the early postoperative period may be advisable to prevent these motions.

Biomechanical consequences of replacement of the anterior cruciate ligament with a patellar ligament allograft. Part I: insertion of the graft and anterior-posterior testing.

Nineteen fresh-frozen knee specimens from cadavera were tested for anterior-posterior laxity with 200 newtons of force applied to the tibia. A cylindrical cap of subchondral bone containing the tibial insertion of the anterior cruciate ligament was isolated with a coring cutter and was potted in acrylic. A thin wire was connected to the undersurface of the cap, and relative displacement between the cap and the tibia was measured with an isometer as the knee was extended. The cap of bone was connected to a load-cell that recorded force in the intact ligament during anterior-posterior testing with the tibia locked in neutral, internal rotation, and external rotation. The anterior cruciate ligament was then resected, and a femoral tunnel was drilled at the site where the isometer readings from the wire were the same as those obtained for the intact anterior cruciate ligament. A bone-patellar ligament-bone graft was used to reconstruct the anterior cruciate ligament, and the isometer measurements were repeated with the graft in place. The graft was pre-tensioned at 30 degrees of flexion to restore normal anterior-posterior laxity. Anterior-posterior laxity tests were repeated at this level of pre-tension (laxity-matched pre-tension) as well as at a level that was forty-five newtons greater (over-tension). The moment required to extend the knee was measured before and after insertion of the graft at both levels of pre-tension. When the tibia was locked in positions of internal and external rotation, the anterior-posterior laxities and the forces in the anterior cruciate ligament (generated by an anterior force applied to the tibia) were significantly less than the corresponding values with the tibia in neutral rotation at 20, 30, and 45 degrees of flexion (p < or = 0.05). Isometer readings for the intact anterior cruciate ligament indicated that the cap of bone retracted into the joint a mean and standard deviation of 3.1 +/- 0.8 millimeters as the knee was extended from 30 degrees of flexion to full extension. For each specimen, the isometer measurements for the trial wire and for the graft were within 1.5 millimeters of those for the intact anterior cruciate ligament. At laxity-matched pre-tension (mean, 28.2 +/- 16.8 newtons), the mean anterior-posterior laxities of the reconstructed knees were within 1.0 millimeter of the corresponding means for the intact knees between 0 and 45 degrees of flexion. Over-tensioning of the graft by forty-five newtons decreased the anterior-posterior laxity a mean of 1.2 millimeters at 30 degrees of flexion. Over-tensioning of the graft did not change the moment required to bring the knee to full extension.

Biomechanical consequences of replacement of the anterior cruciate ligament with a patellar ligament allograft. Part II: forces in the graft compared with forces in the intact ligament.

Seventeen fresh-frozen knee specimens from cadavera were instrumented with a load-cell attached to a mechanically isolated cylinder of subchondral bone containing the tibial insertion of the anterior cruciate ligament. The forces in the intact anterior cruciate ligament were recorded as the knee was passively extended from 90 degrees of flexion to 5 degrees of hyperextension without and with several constant tibial loads: 100 newtons of anterior tibial force, ten newton-meters of internal and external tibial torque, and ten newton-meters of varus and valgus moment. The anterior cruciate ligament was resected, and a bone-patellar ligament-bone graft was inserted. The knee was flexed to 30 degrees, and the graft was pre-tensioned to restore normal anterior-posterior laxity. The knee-loading experiments were repeated at this level of pre-tension (laxity-matched pre-tension) and at a level that was forty-five newtons greater than the laxity-matched pre-tension (over-tension). During passive extension of the knee, the forces in the graft were always greater than the corresponding forces in the intact anterior cruciate ligament. Over-tensioning of the graft increased the forces in the graft at all angles of flexion. At full extension, the mean force in the anterior cruciate ligament was fifty-six newtons; the mean force in the graft at laxity-matched pre-tension was 168 newtons, and it was 286 newtons in the over-tensioned graft. Greater pre-tensioning may be required when the knee demonstrates apparent tightening of the graft in flexion. The mean forces in the graft generated during all constant loading tests were greater than those for the intact anterior cruciate ligament over the range of flexion. When the graft was over-tensioned, the forces generated by the anterior tibial force and by varus and valgus moment increased but those generated by internal and external tibial torque did not. There was no significant change in the mean tibial rotation as a function of the angle of flexion of the knee after insertion of the graft; normal tibial rotation of the knee during passive extension (the so-called screw home mechanism) was eliminated.

Effects of combined knee loadings on posterior cruciate ligament force generation.

Resultant forces in the posterior cruciate ligament were measured under paired combinations of posterior tibial force, internal and external tibial torque, and varus and valgus moment. The force generated in the ligament from a straight 100 N posterior tibial force was highly sensitive to the angle of knee flexion. For example, at 90 degrees of flexion the mean resultant force in the posterior cruciate ligament was 112% of the applied posterior tibial force, whereas at 0 degree, only 16% of the applied posterior force was measured in the ligament. When the tibia was preloaded by 10 Nm of external torque, only 9-13% of the 100 N posterior tibial force was transmitted to the posterior cruciate ligament at flexion angles less than 60 degrees; at 90 degrees of flexion, 61% was carried by the ligament. This "off-loading" of the posterior cruciate ligament also occurred when the tibia was preloaded by 10 Nm of internal torque, but only at knee flexion angles between 20 and 40 degrees. The addition of 10 Nm of valgus moment to a knee loaded by a 100 N posterior tibial force increased the mean force in the posterior cruciate ligament at all flexion angles except hyperextension; this represents a common and potentially dangerous loading combination. The addition of 10 Nm of varus moment to a knee loaded by a 100 N posterior tibial force decreased the mean force in the ligament between 10 and 70 degrees of flexion. External tibial torque (alone or combined with varus or valgus moment) was not an important loading mechanism in the posterior cruciate ligament. The application of internal torque plus varus moment at 90 degrees of flexion produced the greatest posterior cruciate ligament forces in our study and represented the only potential injury mechanism that did not involve posterior tibial force.

Variables affecting pedicle screw plate fixation of an unstable L3-L4 defect.

Fresh frozen human cadaveric spinal specimens (T8-S1) were subjected to pure flexion extension bending moment and pure axial torque loadings while intervertebral rotations were recorded at the L3-L4, L2-L3, and Ll-L2 discs. A standardized unstable defect was created at the L3-L4 disc, and loading tests were repeated after application of bilateral Steffee plates in 2 configurations: a short plate with 2 pedicle screws (spanning the defect) and a longer plate with 3 pedicle screws (spanning the defect and 1 disc above). Each plating configuration was tested in the unlocked state (nuts compressing the plate down onto the spine) and locked state (nuts above and below the plate tightened against each other to clamp the plate to the screws). Locking the plates to the screws had no effect on any intervertebral rotation at any disc level. Use of a longer plate that also spanned the disc above the defect offered no advantage in controlling flexion extension rotations at the defect site. However, mean torsional rotation at the defect site with the 3-screw plate was approximately 50% of the mean for a 2-screw plate. Extension and torsional rotations at the L2-L3 disc (1 level above the defect site) were unaffected by application of a 2-screw plate; flexion rotation at this level increased slightly after plating. All motions at the L2-L3 disc were reduced (as would be expected) when the 3-screw plate spanned this uninjured disc. Plating the defect had no effect on disc rotations at the L1-L2 disc (2 levels above the fracture site).

Combined knee loading states that generate high anterior cruciate ligament forces.

Injuries to the anterior cruciate ligament frequently occur under combined mechanisms of knee loading. This in vitro study was designed to measure levels of ligament force under dual combinations of individual loading states and to determine which combinations generated high force. Resultant force was recorded as the knee was extended passively from 90 degrees of flexion to 5 degrees of hyperextension under constant tibial loadings. The individual loading states were 100 N of anterior tibial force, 10 Nm of varus and valgus moment, and 10 Nm of internal and external tibial torque. Straight anterior tibial force was the most direct loading mechanisms; the mean ligament force was approximately equal to applied anterior tibial force near 30 degrees of flexion and to 150% of applied tibial force at full extension. The addition of internal tibial torque to a knee loaded by anterior tibial force produced dramatic increases of force at full extension and hyperextension. This loading combination produced the highest ligament forces recorded in the study and is the most dangerous in terms of potential injury to the ligament. In direct contrast, the addition of external tibial torque to a knee loaded by anterior tibial force decreased the force dramatically for flexed positions of the knee; at close to 90 degrees of flexion, the anterior cruciate ligament became completely unloaded. The addition of varus moment to a knee loaded by anterior tibial force increased the force in extension and hyperextension, whereas the addition of valgus moment increased the force at flexed positions. These states of combined loading also could present an increased risk for injury. Internal tibial torque is an important loading mechanism of the anterior cruciate ligament for an extended knee. The overall risk of injury to the ligament from varus or valgus moment applied in combination with internal tibial torque is similar to the risk from internal tibial torque alone. External tibial torque was a relatively unimportant mechanism for generating anterior cruciate ligament force.

External fixation in arthrodesis of the ankle. A biomechanical study comparing a unilateral frame with a modified transfixion frame.

Arthrodesis of the ankle was performed on eight fresh-frozen human specimens, and four external fixator configurations were applied: a unilateral external fixator, a Calandruccio clamp alone, a Calandruccio clamp with two anterior struts, and a Calandruccio clamp with four struts (two anterior and two posterior). Each specimen was subjected to 4.0 newton-meters of internal-external tibial torque and 4.0 newton-meters of plantar flexion-dorsiflexion bending moment. The unilateral frame permitted the least tibiotalar motion during plantar flexion-dorsiflexion testing. The Calandruccio clamp allowed more than twice the motion allowed by the unilateral fixator in this testing mode, but the rigidity of the fixation with the Calandruccio clamp was improved with the addition of two interlocking struts, which connected fixator pins proximal and distal to the site of the arthrodesis. The addition of two more struts, to make a four-strut construct, resulted in a small (0.8-degree) additional decrease in the mean tibiotalar motion. There were no detectable differences in the mean tibiotalar motion due to torsional loading between the four fixation configurations.

Direct in vitro measurement of forces in the cruciate ligaments. Part I: The effect of multiplane loading in the intact knee.

Specially designed load-transducers that measured the resultant forces exerted by the posterior and anterior cruciate ligaments on their respective femoral and tibial insertions were applied to eighteen fresh-frozen cadaveric knees for a series of controlled loading experiments. The mean force in the posterior cruciate ligament at 5 degrees of forced hyperextension of the knee was 23 per cent of the mean force in the anterior cruciate ligament. When the knee was hyperflexed by application of 10.0 newton-meters of bending moment to the tibia, the mean force in the posterior cruciate ligament was 55 per cent of that in the anterior cruciate ligament. Quadriceps tendon pull increased the force in the posterior cruciate ligament in twelve of the fourteen specimens to which it had been applied, at 80 and 90 degrees of flexion only. The force generated in the posterior cruciate ligament by applied internal tibial torque was greatest when the knee was in 90 degrees of flexion; the force in the anterior cruciate ligament was greatest when the knee was fully extended. External tibial torque generated force in the posterior cruciate ligament in only eight specimens, and only at 80 and 90 degrees of flexion. The levels of force that were generated in the posterior cruciate ligament by applied varus and valgus bending moment were greatest at 90 degrees of flexion of the knee; the levels of force in the anterior cruciate ligament were greatest with the knee in full extension. With the knee flexed 90 degrees and the tibia in neutral rotation, fifty newtons of applied posterior tibial force increased the mean force in the posterior cruciate ligament by 58.4 newtons; at full extension, no increase in the force in the ligament was recorded, indicating that tensed capsular structures were absorbing the applied load. When the tibia was internally or externally rotated by applied tibial torque, the increases in the force in the ligament from applied posterior tibial force were sharply diminished.

Direct in vitro measurement of forces in the cruciate ligaments. Part II: The effect of section of the posterolateral structures.

Specially designed load-transducers were applied to eight fresh-frozen cadaveric knee specimens in order to measure resultant forces in both cruciate ligaments as the knees were subjected to straight varus-valgus bending moment and to tibial torque (with and without a superimposed posterior tibial force). The forces in the ligaments and tibial rotation were recorded at seven angles of flexion of the knee, between 0 and 90 degrees, before and after section of the posterolateral structures. Ligamentous section increased angulation of the tibia when varus moment was applied to the knee; the large increases in lateral opening of the knee joint were accompanied by increases in the force in the anterior cruciate ligament at all angles of flexion and increases in the force in the posterior cruciate ligament between 45 and 90 degrees of flexion. When valgus moment was applied, there were no significant changes in valgus angulation or the resultant force in either cruciate ligament after ligamentous section. Ligamentous section increased rotation of the tibia when external torque was applied to the knee. The increased external rotation was accompanied by decreases in the force in the anterior cruciate ligament between 0 and 20 degrees of flexion of the knee and increases in the force in the posterior cruciate ligament between 45 and 90 degrees of flexion. In the studies involving applied internal tibial torque, after ligamentous section, rotation of the tibia increased slightly between 60 and 90 degrees of flexion. The force in the anterior cruciate ligament increased between 0 and 20 degrees of flexion, while the force in the posterior cruciate ligament was unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of section of the medial collateral ligament on force generated in the anterior cruciate ligament.

Ten fresh-frozen knees from cadavera were instrumented with a specially designed transducer that measures the force that the anterior cruciate ligament exerts on its tibial attachment. Specimens were subjected to tibial torque, anterior tibial force, and varus-valgus bending moment at selected angles of flexion of the knee ranging from 0 to 45 degrees. Section of the medial collateral ligament did not change the force generated in the anterior cruciate ligament by applied varus moment. When valgus moment was applied to the knee, force increased dramatically after section of the medial collateral ligament; the increases were greatest at 45 degrees of flexion. Section of the medial collateral ligament had variable effects on the force generated in the anterior cruciate ligament during internal rotation but dramatically increased that generated during external rotation; these increases were greatest at 45 degrees. Section of the medial collateral ligament increased mean total torsional laxity by 13 degrees (at 0 degrees of flexion) to 20 degrees (at 45 degrees of flexion). Application of an anteriorly directed force to the tibia of an intact knee increased the force generated in the anterior cruciate ligament; this increase was maximum near the mid-part of the range of tibial rotation and minimum with external rotation of the tibia. Section of the medial collateral ligament did not change the force generated in the anterior cruciate ligament by straight anterior tibial pull near the mid-part of the range of tibial rotation.(ABSTRACT TRUNCATED AT 250 WORDS)

Arthrodesis of the ankle with cancellous-bone screws and fibular strut graft. Biomechanical analysis.

The stability of an arthrodesis with two cancellous-bone screws across the ankle joint was evaluated in eighteen ankles from fresh-frozen cadavera. Tibiotalar motion was recorded in response to the following loading modes: medial-lateral moment, plantar flexion-dorsiflexion moment, and internal-external tibial torque. The series of loading tests was performed with two cancellous-bone screws through the tibia into the talus and a lateral fibular strut graft fixed with a proximal and a distal screw. The tests were repeated after the strut graft was removed, and again after it had been reapplied. The amount of motion at the site of the arthrodesis was greatest with tibial torque and was least with medial-lateral bending; this was true for specimens with or without a fibular strut graft. Removal of the strut graft allowed increased tibiotalar motion for all modes of loading; increases in motion were far greater for specimens of poor bone quality.

Strength of initial mechanical fixation of screw ring acetabular components.

This study was conducted to determine the effects of design on the initial fixation of several types of screw-ring acetabular components. The components were tested in polyurethane foam to assess relative screw fixation strengths with a consistent material. Embalmed pelves from anatomic specimens were used to conduct paired tests between designs that showed large differences in insertional torque to failure in foam. The quality of the initial fixation in foam was found to be dependent on the design features of the components. Components with widely spaced, deep threads, and minimal thread interruptions offered the strongest initial fixation in foam. Tests in bone revealed a wide range of fixation strengths reflecting the variability in bone quality. No differences in fixation strength attributable to component design were observed in bone. When the insertional failure torque was greater than 60 N.m, one-half of the pelves fractured, and these fractures occurred with all designs. At failure torques less than 60 N.m, failure was predominantly due to thread strippage of the screw, with only two of 20 specimens experiencing pelvic fracture.

Direct measurement of resultant forces in the anterior cruciate ligament. An in vitro study performed with a new experimental technique.

A new technique was used to measure the resultant forces in the anterior cruciate ligament during a series of loading experiments on seventeen fresh-frozen cadaver specimens. The base of the ligament's tibial attachment was mechanically isolated with a coring cutter, and a specially designed load-transducer was fixed to the bone-plug that contained the ligament's tibial insertion so that the resultant forces were directly measured by the load-cell. Although the magnitudes of values for forces varied considerably between specimens for a given test condition, the patterns of loading with respect to direction of loading and the angle of flexion of the knee were remarkably consistent. Passive extension of the knee generated forces in the ligament only during the last 10 degrees of extension; at 5 degrees of hyperextension, the forces ranged from fifty to 240 newtons (mean, 118 newtons). When a 200-newton pull of the quadriceps tendon was applied to extend a knee slowly against tibial resistance, however, the force in the ligament increased at all angles of flexion of the knee. Internal tibial torque always generated greater forces in the ligament than did external tibial torque; higher forces were recorded as the knee was extended. The greatest forces (133 to 370 newtons) were generated when ten newton-meters of internal tibial torque was applied to a hyperextended knee. Fifteen newton-meters of applied varus moment generated forces of ninety-four to 177 newtons at full extension; fifteen newton-meters of applied valgus moment generated a mean force of fifty-six newtons, which remained unchanged with flexion of the knee. The force during straight anterior translation of the tibia was approximately equal to the anterior force applied to the tibia. The application of 925 newtons of tibiofemoral contact force reduced the mean force in the ligament that was generated by 200 newtons of anterior pull on the tibia by 36 per cent at full extension and 46 per cent at 20 degrees of flexion.