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.

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.

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.

A dynamic biomechanical study of scapholunate ligament sectioning.

A biomechanical study was performed on fresh cadaver forearms to investigate the role of the scapholunate interosseous ligament in carpal stability. Scaphoid and lunate motion and radiocarpal and ulnocarpal pressure patterns were continually monitored while the wrist was moved physiologically. Prior to ligament sectioning, it was found that the position of the scaphoid and lunate were dependent on both the wrist position and the direction of wrist motion. Sectioning the scapholunate interosseous ligament caused increased scaphoid flexion, scaphoid pronation, and lunate extension. Pressure in the radiocarpal and ulnocarpal joint was redistributed following ligament sectioning. These findings support the clinical impression that the scapholunate interosseous ligament is an important stabilizer of the scaphoid and lunate.

Relative articular inclination of the distal radioulnar joint: a radiographic study.

To determine the congruency of the distal radioulnar joint, 100 standardized normal wrist x-ray films were made and the following measurements taken: ulnar seat inclination, sigmoid notch inclination, and ulnar variance. The inclination angles were different in all but two cases. Relative to the long axis of the ulna, the sigmoid notch inclination averaged 7.7 degrees and the ulnar seat inclination averaged 21.0 degrees. There was a moderate correlation between the two inclination angles as well as between both sigmoid and ulnar seat inclination and ulnar variance. The data show that a wide variation between the inclination of the sigmoid notch and ulnar seat exists, which may explain why symptomatic articular incongruity can occur following ulnar shortening.

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.

A technique for measuring absolute toe pressures: evaluation of pressure-sensitive film techniques.

Although a number of pathologies of the forefoot in ballet dancers on pointe have been described, pressures and deforming forces have not been adequately measured. To evaluate the possible use of pressure-sensitive film (PSF) in measuring the pressures on the external soft tissues in such a confined space as the dancer's toe shoe, it was tested and calibrated with 20 cadaver toes. Each cadaver toe was internally stabilized and loaded longitudinally against PSF on a flat surface. The resultant films were analyzed with a video imaging system and the pressures and total forces were determined. Results showed that the linearity of the PSF to pressure had a regression value of 0.98. By using two sensitivity ranges of films, the total force measured by the PSF was found to be within 10% of the known applied force on each toe. The PSF, therefore, may very well be a useful and accurate method of measuring external soft tissue pressures on the forefoot.

Force transmission through the distal ulna: effect of ulnar variance, lunate fossa angulation, and radial and palmar tilt of the distal radius.

The relationship between the amount of force transmitted through the distal ulna and seven radiologically apparent anatomic parameters (ulnar variance, radial tilt, palmar tilt, lunate fossa angulation, carpal height, carpal ulnar distance, and ulnar head inclination) was examined in 58 fresh cadaver forearms. A positive, although very weak, relationship was found between the amount of force and the ulnar variance (r = 0.44). This suggests that a clinically more positive ulnar variant wrist will not necessarily cause more force to be transmitted to the head of the ulna than a wrist with a more negative ulnar variance, primarily because the triangular fibro-cartilage complex is thicker in arms with a more negative ulnar variance. Changes in ulnar variance of a forearm due to ulnar lengthening or radial shortening do, however, dramatically alter the force transmission. No other relationships were found between the ulnar force and the other radiologic parameters.

Distribution of pressures and forces on the wrist after simulated intercarpal fusion and Kienböck's disease.

Limited intercarpal fusion has been recommended for the treatment of Kienböck's disease. The effects of a simulated scapho-trapezio-trapezoid (STT) fusion and simulated Kienböck's disease on the biomechanics of the radio-ulno-carpal joint were investigated. The percent force, the centroid of the force, and the percent contact area in each fossa were determined. It was found that a STT fusion with the scaphoid in a neutral or extended position unloads the lunate fossa. STT fusion in flexion does not affect lunate load. It is concluded from this biomechanical experiment that STT fusion with the scaphoid in a neutral or extended position unloads the radiolunate joint regardless of the condition of the lunate. This load is shifted to the radioscaphoid articulation.