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.

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.

Biomechanical evaluation of thumb opposition transfer insertion sites.

The optimal location for insertion of the transferred tendon in opposition transfer is controversial. The purpose of this study was to examine 4 commonly used insertion sites into the thumb and determine which maximizes thumb opposition. The flexor digitorum superficialis of the ring finger was used as a donor tendon and was attached in random order to the abductor pollicis brevis (APB) tendon, the APB and extensor pollicis longus, the flexor pollicis brevis (FPB) and dorsal radial extensor hood, and the ulnar extensor hood at the base of the proximal phalanx. As normal opposition was simulated, the minimum distance between the thumb and little finger and the pinch force were measured. The FPB and radial dorsal extensor hood site resulted in the statistically highest pinch force. The FPB and radial dorsal extensor hood and the APB sites had statistically smaller minimum distances between the thumb and little finger than the ulnar extensor hood site. A subjective evaluation of the 3-dimensional thumb path of motion revealed that the FPB and radial dorsal extensor hood site and the APB insertion site allowed the closest approximation of normal thumb opposition. This biomechanical study supports the use of the FPB and radial dorsal extensor hood insertion site or APB insertion site for opposition transfers.

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.

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.

The effect of hyperbaric oxygen on medial collateral ligament healing in a rat model.

Hyperbaric oxygen has been shown to promote healing in bone and some soft tissues. This study was undertaken to determine its effect on ligamentous healing. Forty-eight Sprague-Dawley rats underwent a standardized surgical laceration of the right (divided) medial collateral ligament, whereas the left (undivided) medial collateral ligament was not surgically lacerated. A control group of 24 rats recovered without intervention. An experimental group consisting of the other 24 rats was exposed to hyperbaric oxygen at 2.8 atmospheres for 1.5 hours a day for 5 days after the surgery. Six rats from each group were euthanized at 2, 4, 6, and 8 weeks. The stiffness and final force to failure were recorded for the divided and undivided medial collateral ligaments. At 4 weeks, a statistically greater force was required to cause failure of the previously divided ligaments that had been exposed to hyperbaric oxygen than those that had not. The stiffness and force to cause failure of previously divided ligaments were statistically greater at 4 weeks than at 2 weeks, whether or not hyperbaric oxygen was used. No additional statistical increases in stiffness or force were observed at 6 weeks.

An anatomic study of the ligamentous structure of the triangular fibrocartilage complex.

An anatomic study of the ligamentous structures of the triangular fibrocartilage complex and their attachments on the ulnar styloid was performed using 27 embalmed cadaver wrists. The dorsal and palmar distal radioulnar ligaments of the triangular fibrocartilage complex in each specimen contained a superficial and a deep portion. The deep portion of both ligaments inserted on the fovea of the ulna. The superficial portion of both ligaments surrounded the articular disc uniting at the ulnar-most portion of the articular disc. The tissue that is between the ulnar aspect of the superficial ligament (and integrated on its periphery) and the ulnar capsule is defined as the meniscus homologue. Anatomic variations in the meniscus homologue and the prestyloid recess (the cavity adjacent to the ulnar styloid) were seen in 1 of 3 ways; the narrow opening type in 74% of specimens, the wide opening type in 11%, and the no opening type in 15%. The ulnotriquetral ligament inserted on the palmar-radial aspect of the base of the ulnar styloid and the ulnolunate ligament inserted on the palmar border of the articular disc.

Pressure distribution in the distal radioulnar joint.

Measurement of the pressure distribution within the distal radioulnar joint was performed in fresh cadaver forearms at varying positions of forearm rotation. Axial loads of 0 N, 36 N, and 89 N were applied to the wrist flexors and extensors. At neutral forearm rotation and application of 89 N axial load, an average of 12.5% of the sigmoid notch area was in contact with the ulna. Analysis of the pressure plots reveals that in pronation, the pressure was concentrated in the dorsal portion of the sigmoid notch and that in supination the pressure was distributed in the palmar portion.

Changes in carpal tunnel pressures following endoscopic carpal tunnel release: a cadaveric study.

The purpose of this experiment was to determine the amount of tissue that must be sectioned to adequately decompress the median nerve during an endoscopic carpal tunnel release procedure. In 6 fresh cadaver forearms, 2 balloons were inserted into the carpal tunnel. The first balloon was filled with saline solution to cause an initial carpal intracanal pressure of 50 mmHg. Pressure measurements were recorded, using the second balloon, at various increments of the flexor retinaculum division at 3 wrist positions (neutral, 35 degrees ; flexion, 35 degrees extension). At all increments of sectioning, carpal tunnel pressures in the neutral wrist position were consistently lowest and the values in 35 degrees extension were greatest. At each wrist flexion/extension angle, the pressure statistically decreased during incremental division of the flexor retinaculum. Incomplete release of the transverse carpal ligament resulted in incomplete decompression in the canal. Sectioning the overlying aponeurosis caused a further significant decrease in intracanal pressure.

The relative contribution of selected carpal bones to global wrist motion during simulated planar and out-of-plane wrist motion.

The relative contribution of the scaphoid, lunate, triquetrum, and capitate to wrist motion was examined in 6 fresh cadaver forearms. A wrist-joint motion simulator was used to dynamically move each wrist through planar and nonplanar motions. During wrist flexion-extension, the motion of the capitate closely followed the motion of the third metacarpal, while the lunate motion was approximately 50% of the total motion; the triquetrum, 65%, and the scaphoid, 90%. Similar differences in motion for these carpal bones occurred during radioulnar deviation and circumduction and dart-throw motions. This suggests that the scaphoid, lunate, and triquetrum do not normally function as a single unit, but that each bone has an unique arc of motion during global wrist motion.

Modeling the tensile behavior of the cement-bone interface using nonlinear fracture mechanics.

The tensile mechanical behavior of the cement-bone interface where there was a large process (plastic) zone at the interface was modeled using a nonlinear fracture mechanics approach. A finite element method was employed, which included a piecewise nonlinear interface, to investigate the behavior of experimental cement-bone test specimens and an idealized stem-cement-bone (SCB) structure. The interface model consisted of a linear elastic region with high stiffness until the yield strength was reached, followed by an exponential softening region, until zero stress. The yield strength and rate of exponential softening after yielding at the cement-bone interface were shown to have a marked effect on the structural stiffness of the SCB model. The results indicate that both yield strength and postyield behavior should be included to characterize the mechanics of the cement-bone interface fully.

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.

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.

The interosseous membrane of the forearm: anatomy and function.

The anatomic detail of the interosseous membrane was studied by dissection of 20 preserved cadaveric specimens. The interosseous membrane was found to be a complex structure consisting of a central band, accessory bands, a proximal interosseous band, and membranous portions. The central band, a stout ligamentous structure, was found in all specimens. Fibers of the central band originate on the radius and are oriented distal and ulnar an average of 21 degrees to the longitudinal axis of the ulna. Accessory bands were of less substance than the central band but were present in all specimens. The number of accessory bands ranged from 1 to 5. The proximal interosseous band is located on the dorsal surface only, and its fibers run counter to the central band. It shares a point of origin with the central band on the radius. This structure was present in 17 of 20 specimens. Since the central band was the most dominant and consistent structure, we chose to analyze the strain in the central band in 6 preserved specimens. Maximum strain in the central band of the intact specimen occurs in neutral forearm rotation. Once the radial head is removed, the percent strain universally increases throughout the arc of forearm rotation and peak strain shifts to pronation.

Reconstruction of the interosseous membrane of the forearm in cadavers.

The biomechanical function of the interosseous membrane of the forearm was examined in 12 fresh cadaver forearms. The strain in the central band of the interosseous membrane was found to be greatest in full pronation and was significantly increased with excision of the radial head. The proximal/distal location of the lunate fossa of the radius with respect to the ulna was measured and was found to be most distal in supination and most proximal in pronation, in both the intact specimen and after excision of the radial head. Serial sectioning of the interosseous membrane and the triangular fibrocartilage complex (TFCC) demonstrated that both the central band and the TFCC are important to the axial stability of the forearm. Reconstruction of the central band, using a graft based upon the flexor carpi radialis, was performed in all 12 specimens after the interosseous membrane and the TFCC were sectioned. It was successful in preventing complete migration of the radius to the capitellum, but it was not capable of completely restoring the longitudinal stability of the forearm. Central band reconstruction as described here has not been performed in the clinical setting and is not advocated for clinical application at this time.

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.

The effect of dorsally angulated distal radius fractures on distal radioulnar joint congruency and forearm rotation.

A biomechanical cadaver study was performed to evaluate the effect of dorsally angulated distal radius fractures on the distal radioulnar joint. Frykman I distal radius fractures were simulated, and laxity measurements were taken with and without sectioning the triangular fibrocartilage complex and the interosseous membrane. The findings of this study were threefold. First, measured in terms of radial diastasis, incongruency of the distal radioulnar joint occurred with increasing dorsal tilt of the distal radius. It became most dramatic with a change of more than 20 degrees of dorsal angulation of the distal radius. This corresponds to approximately 10 degrees of dorsal tilt of the articular surface of the distal radius, as measured on an x-ray film. Second, increased dorsal angulation caused interosseous membrane tightness and limited maximum pronation and maximum supination. Third, distal radioulnar joint dislocation did not occur until both the triangular fibrocartilage complex and interosseous membrane were sectioned. These results reveal the importance of anatomic reduction of the distal radius fracture and evaluation of damaged soft tissue structures.

The stabilizing mechanism of the distal radioulnar joint during pronation and supination.

A biomechanical cadaver study was performed to determine the roles of the stabilizing structures of the distal radioulnar joint during pronation and supination. Subluxation and dislocation of the radius with respect to the ulna were evaluated in seven cadaver forearms placed in supination, pronation, and neutral forearm rotation. The amount of subluxation was measured with all structures intact, and after sectioning in various sequences the dorsal and palmar radioulnar ligaments, the distal portion of the interosseous membrane including the pronator quadratus, and the entire interosseous membrane. After sectioning two of any four structures, the distal radioulnar joint remained stable. When the interosseous membrane was disrupted first, the dorsal radioulnar ligament was found to be more important than the palmar radioulnar ligament in stabilizing the distal radioulnar joint in pronation, and conversely the palmar radioulnar ligament was more important than the dorsal radioulnar ligament in supination. Dislocation, and frequently diastasis, occurred only with sectioning of all four structures. This suggests that all four structures contribute to stability of the distal radioulnar joint.