Bone density comparison of selected carpal and tarsal bones: validation for their use in compression fracture fixation studies of scaphoid screws.

INTRODUCTION: To determine if trabecular, total and cortical bone densities of the capitate, navicular, cuboid, and first cuneiform were equivalent to those of the scaphoid, such that these bones could be used in place of the scaphoid in evaluating new headless scaphoid compression screws. METHODS: Fifty scaphoids, capitates, naviculars, cuboids, and first cuneiforms were harvested from fresh frozen cadavers. The trabecular, total and cortical bone densities were measured using pQCT technology and statistically compared. RESULTS: A paired t comparison between paired scaphoids and capitates showed no difference between the trabecular bone densities. However, their total bone and cortical densities were found to be different. An independent measures ANOVA comparison of the five bones, showed no significant difference in mean trabecular density between the capitates, naviculars and first cuneiforms when compared to the scaphoids. However, the mean total and cortical densities of the first cuneiforms were less than the scaphoids and the mean trabecular, total and cortical bone densities of the cuboids were all less than the scaphoids. DISCUSSION: Compression fracture fixation studies of headless compression screws could be conducted using the capitate, navicular, and first cuneiform as models of the scaphoid when the supply of scaphoids is limited.

Orthogonal femoral plating: a biomechanical study with implications for interprosthetic fractures.

OBJECTIVES: This study tests the biomechanical properties of adjacent locked plate constructs in a femur model using Sawbones. Previous studies have described biomechanical behaviour related to inter-device distances. We hypothesise that a smaller lateral inter-plate distance will result in a biomechanically stronger construct, and that addition of an anterior plate will increase the overall strength of the construct. METHODS: Sawbones were plated laterally with two large-fragment locking compression plates with inter-plate distances of 10 mm or 1 mm. Small-fragment locking compression plates of 7-hole, 9-hole, and 11-hole sizes were placed anteriorly to span the inter-plate distance. Four-point bend loading was applied, and the moment required to displace the constructs by 10 mm was recorded. RESULTS: We found that a 1 mm inter-plate distance supported greater moments than a 10 mm distance in constructs with only lateral plates. Moments supported after the addition of a 9- or 11-hole anterior plate were greater for both 10 mm and 1 mm inter-plate distance, with the 11-hole anterior plate supporting a greater moment than a 9-hole plate. Femurs with a 7-hole anterior plate fractured regardless of lateral inter-plate distance size. CONCLUSION: This suggests that the optimal plate configuration is to minimise lateral inter-plate distance and protect it with an anterior plate longer than seven holes. Cite this article: Bone Joint Res 2015;4:23-8.

Pressures in the distal radioulnar joint: effect of surgical procedures used for Kienbock's disease.

Radial shortening and ulnar lengthening are two accepted surgical methods for treating Kienbock's disease. The effect of these procedures on the pressure within the distal radioulnar joint between the ulnar head and the sigmoid notch of the radius was experimentally evaluated in six fresh cadaver forearms. Radical shortening and ulnar lengthening led to increased pressure at the distal radioulnar articulation and caused shifting of the location of the center of pressure distally within the sigmoid notch. Radial displacement of the distal radial fragment at the time of radial shortening, however, decreased the peak pressures. Based on these experimental data, ulnar lengthening and radial shortening can be expected to alter the normal biomechanics of the distal radioulnar joint.

Frequency spectrum analysis of wrist motion for activities of daily living.

A frequency spectral analysis was performed on wrist motion data for 24 activities of daily living (ADLs). Wrist motion was measured using a triaxial electrogoniometer attached to the wrist using tape (for 12 subjects) and pins (for one subject). Results show that the average predominant frequency component of these ADLs was approximately 1 Hz with 75% of the spectral energy less than 5 Hz. The taped-on electrogoniometer, when compared with the pinned electrogoniometer, was adequate for calculating the predominant frequency component and spread of spectral data, but overestimated the magnitudes of the maximum spectral density and total area of the spectral curves. This discrepancy was largest for axial rotation.

Mechanical factors in electrode-induced osteogenesis.

Previous experiments have suggested that mechanical activity of electrode implants may contribute to the observed bone formation that has been attributed to the electrical currents. This was tested by implanting movable and stationary wires in the medullary canal of the rabbit femora or tibiae. The moving implants uniformly showed significant medullary bone formation at the wire, whereas the stationary ones did not. This bone persisted for as long as 8 weeks and was mostly resorbed by 12 weeks as the implants became fixed. The new trabecular bone closely resembled that typically seen at electrically active implants. Implant motion may have a more important role in electrically induced osteogenesis than previously thought.

Cortical and trabecular bone contribute strength to the osteopenic distal radius.

Fractures of the distal radius are common, especially in postmenopausal women, and their prevalence increases with age. Knowledge of the factors that increase the risk of fracture in this metaphyseal region would have predictive and therapeutic implications. Of particular interest in this study were (a) the relative contributions of cortical and trabecular bone to the strength of the distal radius and (b) the best radiographic features to use as strength indicators. In 21 forearms from fresh cadavera (median age at the time of death, 75 years), single photon absorptiometry and quantitative computed tomography were used to determine bone mineral content (BMC), density (BMC/W), and cross-sectional properties of the radius at distal and midshaft sites. Mechanical testing of the forearms then was used to determine the ultimate force and energy to cause the type of fracture that might be caused by a fall on the outstretched hand. Twelve of the 17 tested specimens sustained a fracture of the distal radius, and five sustained a fracture of the scaphoid. In the group of fractures of the distal radius, we found the cross-sectional area and moment of inertia of the cortical shell at the metaphyseal site to be better correlates of strength than the trabecular area and trabecular moment. In contrast, strength correlated much better with trabecular density than with cortical density. Overall, the best correlates of strength were the BMC and BMC/W at either the distal or proximal site. On balance, these results suggest that the thin cortical shell contributes substantially more to the mechanical strength of the distal radius than has been commonly appreciated.

Wrist joint motion simulator.

A computer controlled wrist joint motion simulator has been developed that actively moves forearms from cadavers through cyclic planar flexion-extension motions, planar radial-ulnar deviation motions, and combined motions such as circumduction. Hybrid position-force feedback control algorithms are used to determine the wrist flexor and extensor tendon forces necessary to achieve the desired motions. The simulator was used in a series of 12 fresh cadaver forearms to produce both flexion-extension and radial-ulnar deviation motions and was found to cause repeatable, physiological movements. In these experiments, the extensor tendon forces were greater than those of the flexors, typically by a factor of two.

Mechanical strength of the cement-bone interface is greater in shear than in tension.

The objective of this study was to determine the relative mechanical properties of the cement-bone interface due to tensile or shear loading. Mechanical tests were performed on cement-bone specimens in tensile (n = 51) or shear (n = 55) test jigs under the displacement control at 1 mm/min until complete failure. Before testing, the quantity of bone interdigitated with the cement was determined and served as a covariate in the study. The apparent strength of the cement-bone interface was significantly higher (p < 0.0001) for the interface when loaded in shear (2.25 MPa) when compared to tensile loading (1.35 MPa). Significantly higher energies to failure (p < 0.0001) and displacement before failure (p < 0.01) were also determined for the shear specimens. The post-yield softening response was not different for the two test directions. The data obtained herein suggests that cement-bone interfaces with equal amounts of tensile and shear stress would be more likely to fail under tensile loading.

Tensile strength of the cement-bone interface depends on the amount of bone interdigitated with PMMA cement.

An experimental investigation was performed to (1) determine the general mechanical behavior and in particular, the post-yield behavior of the cement-bone interface under tensile loading, (2) determine where interface failure occurs, and (3) determine if the mechanical properties of the interface could be related to the density of bone at the interface and/or the amount of cement-bone interdigitation. Seventy-one cement-bone test specimens were machined from human proximal femurs that had been broached and cemented using contemporary cementing techniques. The amount of cement-bone interdigitation was documented and the quantitative computed tomography equivalent mineral density (QCT density) of the bone with cement was measured. Specimens were loaded to failure in tension under displacement control and exhibited linear elastic behavior with some reduction in stiffness until the peak tensile stress was reached (1.28 +/- 0.79 MPa). A substantial amount of strain softening (negative tangent stiffness) with an exponential-type decay was found after the peak stress and continued until there was complete debonding of the specimens (at 0.93 +/- 0.44 mm displacement). Interfacial failure most often occurred at the extent of cement penetration into the bone (56% of specimens) or with small spicules of cement left in the bone (38% of specimens). The results showed that the post-yield tensile behavior contributes substantially to the energy required to cause failure of the cement-bone interface, but the post-yield behavior was not well correlated with the amount of interdigitation or density of bone. Linear regression analysis revealed a moderate (r2 = 0.499, p < 0.0001) positive relationship between the tensile strength of the cement-bone interface and the quantity of bone interdigitated with the cement.

A biomechanical analysis of spinal instrumentation systems in thoracolumbar fractures. Comparison of traditional Harrington distraction instrumentation with segmental spinal instrumentation.

A total of 61 biomechanical tests were performed on 25 cadaveric spinal segments to investigate the comparative strengths of three instrumentation systems: 1) conventional Harrington distraction instrumentation (HRI), 2) segmentally wired Harrington distraction rods, and 3) Luque segmental spinal instrumentation (SSI). In type I testing in which axial preload was applied to normal specimens, and then progressive rotation until ultimate failure followed, five of six Harrington systems failed at the bone-metal interface. In contrast, all six Luque SSI vertebral segments disrupted in a location removed from the bone-metal interface. In Type-II testing (six specimens) in which axial loading of experimentally produced unstable burst fractures was applied, the most stable fixation in resisting compressive loads was segmentally wired Harrington distraction rods (P less than 0.001). In Type-III testing (six specimens), there was axial preloading, then progressive rotation applied to translational fracture-dislocations and this showed that the ability to resist torsion was lowest with plain HRI, slightly improved by segmentally wired HRI, and the stiffest system was Luque SSI (P less than 0.05). The three methods of testing cadaveric segments provided a relevant laboratory model for investigation of spinal instrumentation systems in thoracolumbar fracture stabilization. The results compare favorably with other biomechanical studies, information derived from in vitro and ex vivo animal models and clinical experience with failures of fixation. The biomechanical advantages of segmentally wired Harrington distraction instrumentation in resisting axial loads seem to justify this method of fixation in unstable burst fractures. Similarly, the use of Luque segmental spinal instrumentation with L-rods coupled together is the best method of achieving rotational stability in translational injuries (fracture-dislocations). However, the above biomechanical considerations should be balanced against the increased operative time, more exacting technical expertise required, and possible risk of iatrogenic neurologic sequelae in implementing segmental fixation in unstable thoracolumbar fracture management.

Biomechanical comparison of seven internal fixation devices for the lumbosacral junction.

For comparison purposes, seven different internal fixation systems were applied sequentially to each of six fresh-frozen cadaver spines at the lumbosacral junction. The spines were loaded to a maximum flexion bending moment of 12 Newton meters and an extension moment of 6 Nm. Resultant angular motion was recorded with an electrogoniometer. The Synthes and Kaneda devices provided the most rigid fixation in flexion and extension testing. The Luque internal fixation system was as effective as the Kaneda and Synthes systems at limiting extension but was slightly less successful at limiting flexion. The Knodt system was effective at limiting extension but less effective in limiting flexion. The interfacet screws were effective at limiting flexion but less effective at limiting extension. Harrington rods to the sacral alae were the least effective system in both directions. Sublaminar wires did little to improve their performance.

A biomechanical study of the static stabilizing effect of knee braces on medial stability.

The purpose of this project was to determine if commercially available braces could be shown to produce objective evidence of medial stabilization of the knee. Commercially available athletic braces were evaluated for their effect on abduction forces applied to a cadaver knee with no instability and with experimentally created medial instability. Under computer control, abduction forces were applied while simultaneous data were obtained from an electrogoniometer and transducers applied to the anterior cruciate ligament and the superficial medial collateral ligament at 0 degrees, 15 degrees, and 30 degrees of flexion. Our results showed a reduction in abduction angle using functional braces, whereas prophylactic braces demonstrated little or no protective effect.

The effect of knee braces on lateral impact loading of the knee.

Disruption of the medial supporting structures of the knee occurs commonly in contact sports such as American football and lacrosse. A limited number of clinical and laboratory studies currently document the effectiveness of bracing. The purpose of this project was to determine if commercially available bracing could be shown to produce objective evidence of medial stabilization of the knee. Our model involves the use of a cadaver lower extremity with a fixed foot and suspended femur with a free knee and a lateral impact load applied simulating a clipping injury. Force transducers were placed on the ACL and medial collateral ligament (MCL) and an electrogoniometer was attached to the extremity. The prophylactic braces studied had a limited capacity to protect the MCL from direct lateral stress with the knee in full extension. In flexion or with a change in direction of the load, the protective effect is greatly reduced. The functional braces had a capacity to limit abduction and rotational stresses on the MCL in flexion and extension.

The extensor retinaculum of the wrist: an anatomical and biomechanical study.

Through an anatomical and biochemical study, utilizing sixty-five cadaver specimens, the extensor retinaculum of the wrist was examined. It is a complex variable fibrous structure, whose functions are to prevent dorsal bowstringing of the extensor tendons and to prevent radial and ulnar displacement of extensor tendons. We recommend that a portion of the extensor retinaculum be preserved or reconstructed at the time of dorsal wrist surgery.

Intramedullary screw fixation of Jones fractures: a biomechanical study.

The management of proximal fifth metatarsal ("Jones") fractures in athletes has become increasingly more aggressive, despite a lack of biomechanical data in the literature. A cadaver biomechanical study was conducted to evaluate the strength of intramedullary fixation of simulated Jones fractures loaded to failure via three-point bending on a Materials Testing System machine. In a series of eight intact fifth metatarsal control specimens, the force to failure (fracture) was measured for comparison with repaired specimens. Acute fractures were simulated in 10 pairs of feet via osteotomy at the typical fracture location and were fixed with either a 4.5-mm malleolar screw or a 4.5-mm partially threaded, cancellous, cannulated screw, both placed using conventional intramedullary techniques. Force at initial displacement averaged 73.9 N (SD, 64.7 N) for the malleolar screws and 72.5 N (SD, 42.3 N) for the cannulated screws. Force at complete displacement averaged 519.3 N (SD, 226.2 N) for the malleolar screws and 608.4 N (SD, 179.7 N) for the cannulated screws. The force to failure of the intact specimens was significantly greater than the initial and complete forces to failure for the fixed specimens (P < 0.05, independent measures analysis of variance). There was no statistical difference between the average forces at initial displacement or at complete displacement in the fixed metatarsal specimens for the two different types of screws, but the forces at complete displacement for each screw type were significantly greater than the forces at initial displacement (P < 0.05). On the basis of literature review and data generated from this study, it is apparent that the forces necessary to cause displacement of the stabilized Jones fracture are above what would be transmitted within the lateral midfoot during normal weightbearing. The choice of screw and intramedullary technique of fixation is a matter of surgeon preference, because the choice of screw makes no biomechanical difference.

Biomechanics of the elbow and forearm.

The elbow joint functions as a link between the shoulder and wrist providing an exceptional amount of stability and motion. The elbow joint should not be considered a simple hinge joint; rather there are important out-of-plane motions that affect implant design and surgical reconstruction of the elbow. Forearm rotation, which occurs as the radius wraps around the ulna, guided by the soft tissues at the wrist and elbow joints, also contributes to upper extremity mobility. Force transmission in the forearm is a complex interaction of the radius, ulna, and interosseous membrane. Forces at the wrist affect the transmission of force through the forearm to the elbow joint. During some activities, forces at the elbow joint can reach several times body weight. These motion, stability, and force interactions allow for the forearm's function. It should be recognized that an event at the elbow joint affects the forearm and the wrist, and conversely injury or disease at the wrist joint can affect the elbow.

Biomechanical evaluation of operative procedures to treat Kienböck's disease.

Operative procedures used to treat Kienböck's disease have been biomechanically evaluated experimentally. We have shown that joint leveling procedures, such as radial shortening and ulnar lengthening, experimentally unload the ulna and the radial lunate fossa. For wrists with neutral ulnar variance, a lateral opening or medial closing radial wedge procedure unloads the radial lunate fossa. Scapho-trapezio-trapezoidal fusion and scapho-capitate fusion also unload the radial lunate fossa but at the expense of loading the adjacent joints. Neither a capitate-hamate fusion nor a carpal tunnel release alter the radial ulnar carpal joint loading.

Analysis of the kinematics of the scaphoid and lunate in the intact wrist joint.

In conclusion, this study combined three different technologies to simultaneously monitor scaphoid, lunate, and global wrist motion in three dimensions and concurrently collect data on the pressure distribution in the radiocarpal and ulnocarpal joints. This information was collected dynamically in real time while the wrist was moved in reproducible, physiologic cycles of motion. The scaphoid and lunate flex and extend as well as pronate and supinate while the wrist moves in the plane of flexion and extension. There is minimal radial and ulnar deviation of these carpal bones during this motion. During wrist radial and ulnar deviation, the scaphoid and lunate both flex and extend as well as deviate radially and ulnarly. The pressures in the wrist also change as the wrist moves. Pressures in the wrist are not evenly distributed and, during some movements, are localized to specific areas. The data also support the concept that there is a hysteresis effect on both the carpal bones and the pressure distribution patterns while the wrist is moving. The results of this study can provide baseline data to compare with other studies that evaluate various pathologic abnormalities of the wrist joint.

Force transmission through the distal radioulnar carpal joint: effect of ulnar lengthening and shortening.

Although numerous surgical procedures are performed on the wrist joint, little has been written regarding the effect of these procedures on the biomechanics of the wrist. This study has been undertaken to examine the in vitro consequences of ulnar lengthening and shortening. We have measured the axial forces transmitted through the distal radius and ulna and the pressure distribution on the articulating surface of the radius and triangular fibrocartilage complex for the intact wrist, with ulnar lengthening and with ulnar shortening. Lengthening of the ulna by 2.5 mm in the intact wrist increased the force borne by the ulna from 18.4% to 41.9% of the total axial load. Shortening of the ulna by 2.5 mm decreased axial load borne by the ulna to 4.3%. Removal of the articular disc portion of the triangular fibrocartilage complex decreased the load on the intact ulna from 18.4% to 6.2%. The peak pressure at the ulnolunate articulation increased from 1.4 N/mm2 for the unaltered wrist to 3.3 N/mm2 when the ulna was lengthened by 2.5 mm. These results suggest that the biomechanics of the wrist joint can be dramatically altered with relatively small (2.5 mm) changes in ulnar length and by removal of the articular disc portion of the triangular fibrocartilage complex.