Biomechanical evaluation of a new MatrixMandible plating system on cadaver mandibles.

PURPOSE: Current mandibular plating systems contain a wide range of plates and screws needed for the treatment of mandibular reconstruction and mandibular fractures. The authors' hypothesis was that a single diameter screw could be used in all applications in a plating system. Therefore, the purpose of this study was to test if the 2.0-mm locking screws could replace the 2.4-mm screws to stabilize a 2.5-mm-thick reconstruction plate in the treatment of mandibular discontinuity. MATERIALS AND METHODS: Thirty-six fresh human cadaveric mandibles were used: 18 were plated using 2.0-mm locking screws (experimental) and the other 18 were plated using 2.4-mm locking screws (control). Each group was further divided into 3 subgroups based on the site of loading application: the ipsilateral (right) second premolar region, the central incisal region, and the contralateral (left) first molar region. The same ipsilateral (right) mandibular angular discontinuity was created by the same surgeon. The mandible was mounted on a material testing machine. The micromotions between the 2 segments, permanent and elastic displacements, were recorded after incremental ramping loads. The magnitude of screw back-out and the separation between plate and bone were recorded using a laser scanner (resolution, 0.12 mm) before and after the loading applications. The data were processed. Descriptive analyses and a general linear model for repeated measures analysis of variance were performed. RESULTS: There was no statistically significant difference in permanent displacement (mean, 1.16 and 0.82 mm, respectively) between the 2.0-mm and 2.4-mm screw groups. There also was no statistically significant difference in elastic displacement between the 2 groups (mean, 1.48 and 1.21 mm, respectively). Finally, there were no statistically significant differences in screw back-out or separation between plate and bone between the 2 groups. All means for screw back-out and separation between screw and bone for each group were judged within the error of the laser scanning system (<0.12 mm). CONCLUSION: One may anticipate that the mechanical functions of the 2.0-mm locking screws are not different from those of the 2.4-mm screws when a 2.5-mm-thick reconstruction plate is used to reconstruct mandibular angular discontinuity. However, further biomechanical studies (ie, fatigue of screws) are warranted before a randomized clinical trial can be conducted to definitively prove that the 2.4-mm screws can be replaced by 2.0-mm screws.

The optimal skin incision for minimally invasive total hip arthroplasty performed via the anterolateral approach.

Twelve total hip arthroplasty procedures were performed in fresh cadaveric specimens via the anterolateral approach using straight 9-cm incisions placed in 2 different locations with respect to anatomical landmarks. During each procedure, the forces applied to the wound edges by each of the hip instruments and the pressures and strains generated along the wound edges were measured. Pressures ranging from 40 to 190 kPa were developed between the retractors and the wound edges during acetabular reaming and femoral rasping. The resulting strain along the wound edges averaged 28% during acetabular reaming and 34% during femoral broaching (P < .0001). Maximum strains were recorded at the ends of each incision and averaged 58% and 61%, respectively (P < .0001). These results were independent of the anatomical placement of the skin incision. In total hip arthroplasty, the mechanical trauma associated with the procedure is primarily determined by the surgical approach to the hip and the properties of the subcutaneous tissues, and not the anatomical location of the skin incision itself.

The impact of ulnar collateral ligament tear and reconstruction on contact pressures in the lateral compartment of the elbow.

HYPOTHESIS: Complete ulnar collateral ligament (UCL) injury increases articular pressure and reduces contact area compared with the normal intact UCL. UCL reconstruction restores the contact area and contact pressure observed in the native joint. MATERIALS AND METHODS: Six male cadaveric elbows were mounted on a custom jig capable of simulating the 2 critical phases of the throwing motion during pitching. A contact sensor was placed through an anterior arthrotomy into the radiocapitellar joint. Each specimen then underwent valgus loading at 1.75 and 5.25 Nm of torque with the biceps, brachialis, and triceps under axial load in each testing condition. RESULTS: The average valgus laxity in the intact elbow at 90° was 3.7° ± 0.6° at the 5.25 Nm level of torque, which doubled after transection. The reconstruction group demonstrated less laxity (2.4° ± 0.4°) and reduced valgus angulation of the ulna at 5.25 Nm of torque. The transected UCL condition showed peak contact pressure 67% higher compared with the native ligament group at 5.25 Nm of torque. The reconstructed group increased peak articular cartilage pressures by 33% from the native ligament. At 5.25 Nm of torque for the 90° flexion phase, the transected UCL condition showed an average contact pressure of 84% greater than that of the native ligament group. Reconstruction of the UCL restored average articular pressures to within 20% of intact values at 90°. CONCLUSION: UCL injury increases radiocapitellar contact pressures and reduces resistance of the elbow to valgus loading. Contact pressures and valgus laxity can be improved with UCL reconstruction. DISCUSSION: Taken as a whole, the peak pressure data indicate that the reconstruction restores valgus stability and lateral contact pressures to nearly normal levels under the conditions tested. Because the 90 position is the clinically significant position, these laboratory data support the clinical success of the docking procedure.

Anatomic double-bundle anterior cruciate ligament reconstruction: kinematics and knee flexion angle-graft tension relation.

PURPOSE: The purpose of this study was to compare the bundle tension curves and resultant knee kinematics between 2 tensioning protocols in anatomic double-bundle anterior cruciate ligament (ACL) reconstruction. METHODS: Anatomic double-bundle ACL reconstruction was performed in 7 male cadaveric knees. Each graft was tensioned to 22 N under 2 conditions: (1) both bundles tensioned at 20 degrees of knee flexion (20/20 protocol) or (2) posterolateral (PL) bundle tensioned at 15 degrees and anteromedial (AM) bundle at 45 degrees (45/15 protocol). Knee kinematics were recorded in response to anterior and combined rotatory loads in the intact, ACL-deficient, and reconstructed states. Bundle tension was recorded dynamically with knee motion and during each loading test. RESULTS: Tensioning both bundles at 20 degrees of knee flexion resulted in a reciprocal bundle tension pattern that was not statistically different; the PL bundle tension was greater than the AM bundle tension in full extension, and the AM bundle tension was greater than the PL bundle tension from 25 degrees to 120 degrees. In the second tensioning protocol, the AM bundle tension was significantly greater than the PL bundle tension at all flexion angles. Both tensioning protocols restored normal knee kinematics. CONCLUSIONS: Bundle-tensioning protocol is a variable that has a significant effect on the bundle-loading patterns in double-bundle ACL reconstruction. The 20/20 protocol resulted in AM and PL bundle-loading patterns that were equivalent during dynamic testing, whereas the 45/15 protocol led to excessive tension in the AM bundle in full extension. We recommend equal tensioning of both bundles with the knee at 20 degrees of flexion to restore relatively normal tension curves in each bundle and to avoid excessive stress on the AM bundle. CLINICAL RELEVANCE: In double-bundle ACL reconstruction, there is no consensus regarding bundle-tensioning protocols. This study provides data on the individual bundle tension curves that result from 2 commonly used tensioning protocols. These data will assist clinicians as the technique and application of double-bundle ACL reconstruction move forward.

Screw versus plate fixation of proximal first metatarsal crescentic osteotomy.

BACKGROUND: Hallux valgus associated with metatarus primus varus is a deformity that disrupts normal foot function. Standard treatment has often included distal or proximal metatarsal osteotomy with or without a distal soft tissue procedure. The intrinsically unstable proximal crescentic osteotomy relies on operative fixation for stability. This study examined the strength of fixation of a single screw versus a plate for stabilization of proximal first metatarsal crescentic osteotomies for correction of hallux valgus. MATERIALS AND METHODS: A crescentic osteotomy was performed on nine pairs of fresh/frozen cadaveric feet. The distal fragment was rotated laterally and the osteotomy was temporarily stabilized with a Kirschner wire. The osteotomized metatarsal fragments were fixed with a cancellous screw on one side and single plate on the contralateral side. Each specimen was loaded in a mechanical testing machine, and its response was measured by monitoring the opening of the osteotomy and change in alignment of the fragments. Position and alignment of the proximal and distal fragments were calculated. RESULTS: All of the measures of fixation strength were statistically greater in specimens treated with a plate rather than with screw fixation. Plate fixation provided approximately twice the resistance to disruption of the osteotomy under cyclic loading conditions. CONCLUSION: The dorsal plate is biomechanically more stable than a single cancellous screw when applied to proximal crescentic osteotomies. CLINICAL RELEVANCE: The greater stability of the plate construct may be helpful in selecting the fixation device for these osteotomies.

Comparison of isometric and anatomic reconstruction of the medial patellofemoral ligament: a cadaveric study.

Recent surgical procedures designed to correct recurrent posttraumatic lateral patellar instability focus on reconstructing the medial patellofemoral ligament. This study evaluated and compared patellofemoral kinematics of isometric and anatomic medial patellofemoral ligament reconstructions. Using an infrared motion capture analysis system, patellar tracking was evaluated in the coronal plane in 6 cadaveric specimens. Reconstruction of the medial patellofemoral ligament using an isometric technique did not restore normal patellar tracking at any flexion angle; however, reconstruction using an anatomic technique restored statistically normal patellar tracking from maximal knee extension to 28 degrees of flexion. Neither technique was able to restore normal kinematics in deeper angles of knee flexion.

Does Capsular Laxity Lead to Microinstability of the Native Hip?

BACKGROUND: Hip "microinstability" is commonly cited as the cause of symptoms that occur in the presence of translation of the femoral head away from conformity with the acetabular fossa. However, there is still no consistent objective criteria defining its presence and biomechanical basis. One hypothesis is that abnormal motion of the articular surfaces occurs because of capsular laxity, ultimately leading to clinical symptoms. PURPOSE: To determine the relationship between capsular laxity and abnormal rotation and translation of the hip. STUDY DESIGN: Controlled laboratory study. METHODS: Eight cadaveric hips were dissected down to the capsule and mounted in a customized multiaxial hip activity simulator. Each specimen was loaded with 5 N·m of internal and external rotational torque in full extension and 0°, 30°, 60°, and 90° of flexion. During testing, the relative position and rotation of the femur and the pelvis were monitored in real time with a 6-camera motion analysis system. The testing was repeated after capsular laxity was generated by placing a regular array of incisions ("pie crusting") in the iliofemoral, pubofemoral, and ischiofemoral ligaments. Joint rotation and femoral head translation were calculated with specimen-specific models. A hip microinstability index was defined as the ratio between the length of the locus of the femoral head center and the radius of the femoral head during rotation from extension to 90° of flexion. RESULTS: In intact hips, the components of femoral head translation were within 0.5 mm in positions close to neutral (<30° of flexion). Capsular modification led to significant increases in internal and external rotation ( P < .01) and in the translation of the femoral head center at different positions ( P < .05). Compared with intact hips, the femoral head was inferiorly displaced during external rotation and anteroinferiorly during internal rotation. The length of the locus of the femoral head center increased from 3.61 ± 1.30 mm to 5.35 ± 1.83 mm for external rotation ( P < .05) and from 6.24 ± 1.48 mm to 8.21 ± 1.42 mm for internal rotation ( P < .01). The correlations between rotational laxity and the total translation of the femoral head were not significant, with coefficients of 0.093 and 0.006 in external and internal rotation, respectively. In addition, the hip microinstability index increased from 0.40 ± 0.08 for intact hips to 0.55 ± 0.09 for modified hips ( P < .01). CONCLUSION: The native hip approximates a concentric ball-and-socket joint within 30° of flexion; however, beyond 30° of flexion, the femoral head translation reached as high as 4 mm. Capsular laxity leads to microinstability of the hip, as indicated by significantly increased joint rotations and femoral head translations and an abnormal movement path of the femoral head center. However, there was no correlation between rotational laxity and the increase in femoral head translation. CLINICAL RELEVANCE: Capsular laxity alters normal kinematics (joint rotation and femoral head translation) of the hip, potentially leading to abnormal femoral-acetabular contact and joint degeneration.

Effect of lateral meniscal root tear on the stability of the anterior cruciate ligament-deficient knee.

BACKGROUND: Meniscal root tears are an increasingly recognized subset of meniscal injury. The menisci are critical secondary stabilizers of the anterior cruciate ligament (ACL). The kinematic effect of lateral meniscus posterior root tear in the setting of ACL injury is not known. PURPOSE/HYPOTHESIS: The purpose of this study was to determine the effect of tear of the lateral meniscal root on stability of the ACL-deficient knee. The hypothesis was that disruption of the lateral meniscal root will further destabilize the ACL-deficient knee during a simulated pivot shift. STUDY DESIGN: Controlled laboratory study. METHODS: Pivot-shift testing of 8 fresh-frozen cadaveric knees was performed after attachment of photoreflective flags and preparation of CT scans. Each knee was mounted in a custom activity simulator and dynamically loaded from 15° to 90° of flexion with all the permutations of the following: iliotibial band force (50, 75, 100, 125, 150, and 175 N), internal rotation moments (1, 2, and 3 N·m), and valgus moments (5 and 7 N·m). In addition, anterior stability tests were performed by applying a 90-N anterior force to the tibia at flexion angles of 15°, 30°, 45°, 60°, and 90°. During each test, the anterior tibial translation and rotation of the tibia were measured with a high-resolution multiple infrared camera motion analysis system for the following 3 conditions: ACL-intact (ACL-I), ACL-deficient (ACL-D), and ACL-deficient/lateral meniscal posterior root avulsion (ACL-D/LMR-A). RESULTS: A pivot-shift phenomenon was observed in the ACL-D and ACL-D/LMR-A conditions. The mean tibial translation of the lateral tibial condyle during the pivot-shift maneuver was 2.62 ± 0.53 mm for the ACL-I knees, 6.01 ± 0.51 mm for the ACL-D knees (P value vs. intact: .0005), and 8.13 ± 0.75 mm for the ACL-D/LMR-A knees (P value vs intact: <.0001). During the pivot-shift maneuver, translation was significantly increased in the ACL-D/LMR-A condition compared with the ACL-D condition (P = .0146). Compared with the intact group, anterior tibial translation during the Lachman maneuver also increased at 30° and 90° of flexion in the ACL-D group (P < .0001) and the ACL-D/LM group (P < .0001). No statistically significant difference was found between the ACL-D and ACL-D/LMR-A groups during the Lachman maneuver at 30° and 90° (P = .16 and .72, respectively). CONCLUSION: A tear of the lateral meniscal posterior root further reduces the stability of the ACL-deficient knee during rotational loading. CLINICAL RELEVANCE: This study shows that lateral meniscal root injury further destabilizes the ACL-deficient knee and thus advances the concept that the lateral meniscus is a secondary stabilizer of the knee under pivot-shift loading. In the absence of stronger evidence, the study data suggest a rationale for surgical repair of lateral meniscal root tears encountered in the setting of ACL tears.

Backside wear of modular ultra-high molecular weight polyethylene tibial inserts.

BACKGROUND: The capture mechanisms of modular tibial total knee components may allow relative micromotion between the insert and the base-plate, leading to wear at the nonarticulating (backside) surface. Although retrieved components often display laxity in the capture mechanism in the unloaded condition, the magnitude of the relative motion that actually occurs under physiologic conditions has not been determined. This study was performed to assess the impact of different modes of knee-loading on the relative micromotion between the insert and the base-plate and the relationship between the duration that the implant had been in situ and the severity of backside wear. METHODS: Twenty-one posterior-stabilized total knee replacements of one common design (Insall-Burstein II) were retrieved at one to 100 months after implantation. The extent and severity of backside wear was graded with use of stereomicroscopy. All components were soaked in a bath (of physiologic saline solution at 37 degrees C for four days prior to reassembly. The relative micromotion between the insert and the base-plate of each specimen was measured in vitro in two different conditions: with no axial load and with a combination of loads and torques simulating the stance phase of gait. RESULTS: The capture mechanism laxity between the insert and the tibial base-plate in the unloaded condition was approximately eight times larger than the micromotion measured during simulated gait. The capture mechanism laxity allowed a mean (and standard deviation) of 618 +/- 226 micro m of total relative micromotion compared with 103 +/- 54 micro m of relative micromotion during the gait cycle. Under both loading conditions, the predominant direction of interface motion was medial-lateral. No correlation was found between the magnitude of capture mechanism laxity and the relative micromotion measured during simulated gait (p = 0.11). Larger polyethylene protrusions on the backside surface did not correlate with less micromotion (p = 0.48) or with capture mechanism laxity (p = 0.06). CONCLUSIONS: For the implant design that was studied, capture mechanism laxity between the modular insert and the base-plate in the unloaded condition was an order of magnitude larger than and not indicative of the micromotion that occurred during simulated physiologic loading. In addition, polyethylene protrusions into the screw-holes of tibial base-plates did not seat or lock the insert in place and reduce relative motion. CLINICAL RELEVANCE: While some clearance between the insert and the base-plate is required to allow assembly of modular tibial components at the time of surgery, the amount of relative interface motion during a functional activity such as normal gait, which can produce potentially damaging wear debris, is unknown. However, the compressive forces applied to the articular surface during a functional activity may substantially reduce micromotion between the insert and the base-plate relative to the unloaded condition.

A biomechanical comparison of repair techniques for type II SLAP lesions.

BACKGROUND: Multiple options exist for repair of superior labral tears. PURPOSE: To compare commonly used fixation techniques for superior labral tears. STUDY DESIGN: Biomechanical cadaveric study. METHODS: A comparison of the initial strengths of fixation for type II superior labral anterior posterior (SLAP) lesions was performed in three cadaveric shoulder groups, each containing seven specimens. Two groups were repaired with screw-in anchors; one group had vertical sutures, the other horizontal. Group 3 was repaired using bioabsorbable tacks. Cyclic traction was applied to the biceps tendon. Repair failure (2 mm of permanent displacement) and ultimate failure were measured. RESULTS: Specimen stiffness was similar between groups. The mean load to repair failure was 123 +/- 17 N in group 1, 114 +/- 11 N in group 2, and 95 +/- 13 N in group 3. The mean load to ultimate failure was 163 +/- 15 N, 161 +/- 12 N, and 145 +/- 12 N, respectively. Although the repair failure loads of groups 1 and 2 were 29% and 17%, respectively, greater than the tack group, the differences were not statistically significant (P >.05). All ultimate failures occurred at the labral-implant interface. CONCLUSION: Initial fixation strength of tissue tack and suture anchor repairs of SLAP lesions are comparable.

Multistranded hamstring tendon graft fixation with a central four-quadrant or a standard tibial interference screw for anterior cruciate ligament reconstruction.

BACKGROUND: Tibial fixation of hamstring tendon grafts has been the weak link in anterior cruciate ligament reconstruction. HYPOTHESIS: Use of a central four-quadrant sleeve and screw provides superior fixation when compared with standard interference screw fixation. STUDY DESIGN: Controlled laboratory study. METHODS: In eight pairs of cadaveric knees each anterior cruciate ligament was reconstructed using either an interference screw or a central sleeve and screw on the tibial side. The specimens were then subjected to cyclic loading followed by a load-to-failure test. RESULTS: The load required to cause 1 and 2 mm of graft laxity, defined as the separation of the femur and the tibia at the points of graft fixation, was significantly greater with the sleeve and screw than with the interference screw (at 2 mm: sleeve and screw, 216.1 +/- 30.1 N; interference screw, 167.0 +/- 33.2 N). The force at initial slippage for each of the graft strands was significantly higher with use of the central sleeve and screw. CONCLUSIONS: The four-quadrant sleeve and screw device may provide greater surface area for healing of hamstring tendon grafts and allow equal tensioning of graft strands before fixation. These factors are associated with increased strength of fixation and reduced laxity of the graft after cyclic loading. CLINICAL RELEVANCE: Use of the central four-quadrant sleeve and screw system offers increased strength of fixation in anterior cruciate ligament reconstruction with hamstring tendon graft.

The effect of strand configuration on the tensile properties of quadrupled tendon grafts.

PURPOSE: To compare selected biomechanical properties of 4 different quadrupled tendon graft preparations applicable for anterior cruciate ligament reconstruction. TYPE OF STUDY: Biomechanical investigation in a cadaver model. METHODS: Graft strands were arranged in parallel, braided, bolo-plait, and twist configurations. Elongation of the graft was monitored during loading with an infrared optical measurement system, which measured the 3-dimensional position of twelve spherical reflective markers mounted within the camera field. Permanent elongation, stiffness, and ultimate strength of each graft configuration were determined. RESULTS: Permanent graft strain after the first incremental loading cycle was 7.2% in braid, 6.6% in twist, 6.5% in bolo-plait, and 3.3% in parallel constructs (P <.0001). Permanent graft strain after a second incremental loading cycle was 1.2% in bolo-plait, 1.1% in braid, 0.7% in twist, and 0.7% in parallel preparations (P =.0012). Braid and bolo-plait grafts were less stiff than twist or parallel grafts (P <.0001). Differences in failure load were not significant (P =.171). CONCLUSIONS: Variation in graft configuration from the conventional, parallel strand pattern lead to increased viscoelasticity and permanent elongation of grafts with repetitive loading. This deficit is expected to cause increased laxity of the reconstructed knee.

The effect of delayed autogenous bone grafting on the healing of diaphyseal defects in a canine model.

Bone grafting of osseous defects is often delayed to minimize the risk of infection, however, the effect of this delay on defect healing is not clear. Unilateral oblong unicortical diaphyseal femoral defects (30 x 4.5 mm) were created in 3 groups of 18 adult canines that were grafted with autogenous bone immediately, at 2 weeks, and at 6 weeks. After an 8-week healing period, paired femurs, h consisted of defect versus intact specimens, were assessed by biomechanical testing and microradiography. No differences were demonstrated biomechanically or radiographically between the healing defects with immediate versus delayed bone grafting. This study suggests that within the first 6 weeks postoperatively osseous defects can be grafted with autologous bone without adversely affecting bone graft incorporation.

Etiology and severity of impingement injuries of the acetabular labrum: what is the role of femoral morphology?

Injuries to the acetabular labrum have been seen in association with femoroacetabular impingement, but recent studies have reported labral pathology in patients with normal hip morphology. The hypothesis of the current study was that labral lesions could occur without femoroacetabular impingement but that labral pathology would occur more commonly and more severely in hip joints that exhibit reduced head-neck offset. The presence, location, and severity of labral injury were recorded in 22 cadaveric specimens. Computed tomography was used to define the anatomic parameters of proximal femoral morphology. Three-dimensional modeling was used to simulate hip positions that typically cause labral impingement, including high flexion and internal rotation. Femoral morphology was compared between specimens with and without labral pathology using descriptive statistics. Labral pathology was seen in 15 of 22 specimens and was located in the anterosuperior portion of the labrum. No difference existed in age, femoral neck shaft angle, anteversion, acetabular depth, head diameter, alpha angle, or beta angle between specimens with and without labral pathology. The severity of labral pathology correlated with the alpha angle of the proximal femur. This study demonstrates that damage to the labrum may occur in hips with normal proximal femur morphology. However, the findings also indicate that the presence of morphologic features that increase the risk of impingement may predispose the hip joint to a characteristic pattern or severity of labral pathology. The results confirm the importance of considering both femoral morphology and athletic-type activities of the hip when determining the mechanism responsible for injury of the acetabular labrum.

Tensile strain in the anterior part of the acetabular labrum during provocative maneuvering of the normal hip.

BACKGROUND: Injury of the acetabular labrum is a well recognized cause of hip pain in the young, active patient. The exact mechanism of these injuries remains a subject of speculation, although femoroacetabular impingement and twisting maneuvers have both been proposed as critical factors. We examined the hypothesis that torsional maneuvers of the morphologically normal hip joint generate mechanical strain within the acetabular labrum, particularly in areas that are prone to injury. METHODS: Seven human cadaver specimens were loaded during five separate maneuvers with external rotation or abduction torques applied to the hip in neutral alignment and in moderate flexion or extension. Tensile strain within the acetabular labrum was measured with use of the technique of roentgen stereophotogrammetric analysis. RESULTS: Substantial tensile strains were generated within the labrum during each of the loading maneuvers, with no significant difference in strain being noted between the maneuvers. Maximum strain in the anterior part of the labrum averaged 13.6% +/- 7.8% in the axial direction and 8.4% +/- 3.0% in the circumferential direction. The highest mean and maximum strain values were found at the two o'clock position of the labrum, with the highest strain concentration at the bone-labrum interface. CONCLUSIONS: External rotation and abduction maneuvers of the morphologically normal human hip joint in moderate flexion or extension can generate substantial tensile strains in the anterior part of the acetabular labrum. This finding supports the hypothesis that injury to the anterior part of the labrum may occur from recurrent twisting or pivoting maneuvers of the hip joint in moderate flexion or extension without femoroacetabular impingement.

The 2007 Frank Stinchfield Award. The biomechanics of the hip labrum and the stability of the hip.

We explored the mechanical factors leading to the formation of labral tears and the effect of these lesions on hip kinematics at the extremes of joint motion. Using a 3D motion analysis system, the stability of six cadaveric hips was measured during loading maneuvers known to impose anterior loads on the joint margin. These measurements were repeated following venting of the capsule, and after creation of a 15-mm tear in the intact labrum. Compared to the intact hip, 43% and 60% less force was required to distract the femur by 3 mm after venting and creation of a tear. An ER torque of 177 in-lbf in 30 degrees of flexion caused the vented and torn specimens to rotate 1.5 degrees +/- 2.7 degrees and 7.1 degrees +/- 4.7 degrees more than the intact specimen, and the femoral head to displace 1.21 +/- 0.53 mm and 0.67 +/- 0.35 mm, respectively. A breach of the integrity of labral function is shown to lead to decreased femoral stability relative to the acetabulum during extreme ranges of motion.

Components of laxity in interference fit fixation of quadrupled hamstring grafts.

We tested in vitro 15 Poly-L-lactic acid and 14 titanium interference screws in male human cadaver tibia-hamstring constructs. Elongation of the graft, displacement of the screw in the tibial tunnel, and slippage at the graft/screw interface were measured as outcomes with an infrared optical system. All constructs failed by slippage of the graft past the interference screw. On average, graft slippage accounted for 92% of total construct laxity at 5mm total construct elongation, whereas permanent stretching of the midsubstance of the graft accounted for only 8%. Movement of the interference screw in the tibial canal was minimal, averaging 0.01 (0-0.5) mm at construct failure. Inadequacy of isolated tibial interference screw fixation of soft tissue grafts may be overcome if early failure at the graft/screw interface is prevented.

The optimal strategy for stable tibial fixation in revision total knee arthroplasty.

In revision total knee arthroplasty, the optimal strategy for stabilizing the tibial component in the face of proximal defects remains controversial. The stability of a revision tibial prosthesis was measured using a mechanical surrogate of the revision tibia, while varying the defect treatment proximally and the cortical engagement distally. An offset axial load in combination with an axial torque were applied to each specimen to simulate the stance phase of gait. It was found that, in revision total knee arthroplasty, the mechanical stability of tibial fixation is increased by the addition of a canal filling stem, especially in the presence of poor proximal bone. Proximal tibial coverage, even with a custom-fitted tibial component, adds little additional stability to a tibial tray stabilized by intramedullary engagement of a canal-filling stem.