A Biomechanical Comparison of Fixation Techniques in Metacarpal Shaft Fractures.

Background Typically, metacarpal shaft fractures are treated with closed reduction percutaneous pinning, intramedullary nails, or plate fixation. Recently some surgeons have begun using intramedullary headless compression screws. Questions/Purposes The purpose of this study was to compare intramedullary screw fixation to K-wire fixation, which is the standard of care in a transverse metacarpal midshaft fracture, using a cadaveric model. Our hypothesis was that intramedullary screw fixation would have a biomechanical advantage (higher stiffness and peak load to failure) when compared with dual Kirschner wire fixation of transverse metacarpal shaft fractures. Methods Four-point bend testing was performed to compare stiffness and failure load values of seven paired 2nd and 3rd metacarpals instrumented with headless intramedullary compression screw fixation or Kirschner wire fixation. Similar testing was performed on 14 unpaired 4th metacarpals. Results There was no significant difference in peak load ( p = 0.60) or stiffness ( p = 0.85) between fixation groups for the 2nd and 3rd instrumented metacarpals. For the instrumented 4th metacarpals, there was no significant difference in peak load ( p = 0.14), but the stiffness was significantly greater ( p = 0.01) for the compression screw group compared with the Kirschner wire fixation. Conclusions/Clinical Relevance In this study, the load to failure was not different between the two fixation methods and likely both techniques can sustain physiologic loads needed for rehabilitation. The greater stiffness in the 4th metacarpal compression screw group may be related to the smaller canal morphology than in the 2nd and 3rd metacarpals. Larger diameter screws may be needed to obtain a better fit particularly in the 2nd and 3rd metacarpals.

First Carpometacarpal Joint Motion and Proximal Migration of the First Metacarpal After Tensioning of a Suture Device Suspensionplasty Compared With Trapeziectomy: A Biomechanical Cadaver Study.

PURPOSE: The purpose of this cadaveric biomechanical experiment was to evaluate the effects of suture button suspensionplasty of the first carpometacarpal joint on thumb biomechanics and thumb position compared with an intact, arthritic specimen. METHODS: Six tendons in 8 cadaver hands were loaded to simulate 6 activities of daily living and passively moved through a circumduction motion. Proximal migration of the base of the first metacarpal was measured using optical motion sensors in the intact hand, after trapeziectomy, and following insertion of a suture button suspensionplasty with nominal tightening (approximately 4.5 N) and with firm tightening (approximately 44.5 N). RESULTS: Removal of the trapezium caused a significant increase in the proximal migration of the first metacarpal during a simulated jar grasp, opposition, flexion, extension, and abduction (average, 9.5 mm) compared with its location with the thumb in the intact, neutral position (average, 3.8 mm). Firm tightening of the tightrope caused a near elimination of the proximal migration of the first metacarpal (average, 0.7 mm). In all 6 static loading cases with the trapezium removed, firm tightening caused a significantly smaller migration than in the absence of tightening. CONCLUSIONS: This biomechanical cadaver study supports the hypothesis that trapeziectomy results in proximal migration of the first metacarpal. Suture suspensionplasty mitigates against this migration while maintaining normal motion of the first metacarpal compared with the intact state. Firm tightening of the suture does not adversely affect the first metacarpal's mobility and further decreases proximal migration. However, firm tightening may cause impingement between the first and second metacarpals. CLINICAL RELEVANCE: Suture button suspensionplasty can be used in addition to trapeziectomy in the treatment of basal joint arthritis, and may diminish the need for ligament reconstruction or temporary K-wire insertion.

Effect of Various Wrist and Forearm Motions on Distal Radius Forces.

Background Dart throw motions are frequently used during rehabilitation but the ideal orientation of a dart throw motion is unknown. Questions/Purposes The purpose of this study was to measure the axial force on the distal radius during different dart throw motions with the wrist and forearm in various positions. Our hypothesis was that there would be a significant difference on the axial force between various forearm positions and different dart throw orientations. Methods Eight fresh frozen cadaver wrists were moved through 10 different orientations of a dart throw motion with the forearm in neutral, in pronation, and during a dynamic forearm rotation motion while the axial force was measured. Results Significantly smaller axial force occurred with the forearm in pronation than during the dynamic forearm motion. The shorter dart throw motions which were oriented equally toward the flexion/extension and radioulnar deviation axes had significantly smaller distal radius forces than nearly all of the other dart throw motions. Conclusion/Clinical Relevance Rehabilitation protocols incorporating a dart throw motion may be of benefit after injury or surgery. To minimize the axial force transmitted through a healing distal radius fracture, short dart throw motions, oriented at 45 degrees from the sagittal and coronal planes, with the forearm in pronation, might be preferable for range of motion activities during rehabilitation.

Determination of the Force Required to Produce Stab "Wounds" in Fruit Compared With Cadaveric Chest Tissue.

ABSTRACT: In laymen's terms, it can be difficult to communicate how much force is needed to create a stab wound into a person's chest. Previous work has determined the force to puncture the skin, rib cartilage, or bone but without putting the results in terms that are easy to understand.The purpose of this study was to determine the force needed to puncture 3 types of fruit using 3 different types of knives, namely, a steak knife, a butcher's knife, and a lock-blade knife, to help put these previous results in perspective.There was wide variation in the force required to insert a knife into different fruits, but no force for any knife at any location for all fruits exceeded 93.1 N. Results show that force needed to penetrate the skin and allow for an 8-cm blade penetration into the chest is similar to the force required to insert a steak knife for a 6-cm distance into a cantaloupe. In addition, the force needed to penetrate the cartilage is most similar to stabbing a watermelon to 6 cm with a butcher knife. However, the forces required to penetrate the bone are greater than those required to penetrate any fruit with any type of blade.

Biomechanical Comparison of Dart-Throw Motions after Partial Wrist Fusions.

Background Multiple partial wrist fusions exist for the management of arthritic disease. Limited information is available on their effect on wrist range of motion in the dart-throwing direction of wrist motion, even though it is used in most activities of daily living. Purpose The purpose of this study was to measure the retained motion for different orientations of dart-throwing motion for seven different partial wrist fusions and proximal row carpectomy (PRC). Methods Eight fresh frozen right cadavers were tested with the wrist intact and followed simulated fusions. Fusions were performed using an external fixation technique and included scaphocapitate, scapholunate (SL), capitolunate, radiolunate, radioscapholunate, scaphotrapeziotrapezoid, 4 corner fusion, and PRC. Results In the intact wrist, the average arc of wrist motion with the wrist oriented at 20 degrees away from the flexion-extension axis was significantly larger than at any other orientation of motion. All partial wrist fusions and the PRC had significantly smaller average dart-throw arc of motion compared with intact at an orientation 20 and 25 degrees away from flexion-extension. The SL fusion provided a significantly larger arc of motion than most of the other fusions at most orientations. Conclusion/Clinical Relevance This study provides a comprehensive compilation of the range of motion in a functional plane, "the dart-throw motion," for limited wrist fusions and PRC. These data provide the clinician with important information that can be used to educate patients regarding expectations after surgery.

Design Requirements for Scapholunate Interosseous Ligament Reconstruction.

Background As numerous repairs, reconstructions, and replacements have been used following scapholunate interosseous ligament (SLIL) injury, there is a need to define the structural requirements for any reconstruction or replacement. Methods Research has been conducted on the force needed to keep the scaphoid and lunate reduced following simulated injury, the failure force of the native SLIL and various replacements, the stiffness of the SLIL and replacements, and the torsional resistance of the scaphoid relative to the lunate. Results Forces on the order of 50 N are needed to keep the scaphoid and lunate reduced during simple wrist motions in the chronically injured wrist. Even greater forces (up to 110 N) are needed to keep the bones reduced during strenuous activities, such as pushups. The failure force of the entire SLIL has been reported to be as high as 350 N and the failure force of just the dorsal component of the SLIL to be 270 N. Conclusions The design requirements for a reconstruction or repair may vary depending upon the demands of the patient. In a high demand patient, a reconstruction needs to support the above-mentioned forces during cyclic loading (50 N), when performing strenuous activities (110 N), or during a fall (at least 350 N). Any artificial replacement must undergo careful biocompatibility testing.

Determination of Force Required to Produce Stab Wounds in Cadaveric Chest Tissues.

ABSTRACT: When investigating deaths due to stab wounds, a common question asked to the forensic pathologist concerns the force required to inflict a given wound.In this study, tests were performed on 6 human cadavers. A material testing machine was used to produce the stab wounds and to record the force required to penetrate skin, muscle, cartilage, and rib bone of the chest. Three different blades were used: a steak knife, a butcher knife, and a lock-blade knife. On each cadaver, chest injuries were produced at the following locations: (a) skin, intercostal soft tissues; (b) skin, muscle, and cartilage; and (c) skin, muscle, and bone. After the experiment, a chest dissection was performed to confirm the correct locations of the produced stab wounds.The force required to insert a knife into cartilage or bone was significantly greater than the force to insert it into a region only covered by skin. There was wide variation in the force required to insert a knife into different bodies, but no force for any knife at any location for all bodies was greater than 261 N.This study allowed us to obtain quantitative measures of the force required to penetrate human chest tissues, removing subjective factors.

Carpal Motion in Chronic Geissler IV Scapholunate Interosseous Ligament Wrists.

PURPOSE: This study evaluated the biomechanics of Geissler IV (G4) wrists in cadavers and compared them with intact specimens after multiple ligament sectioning to create scapholunate instability. It also evaluated carpal motion changes after sectioning of the lunotriquetral interosseous ligament (LTIL). METHODS: Eight cadaver wrists determined to be G4 arthroscopically were tested using a wrist joint motion simulator. The LTIL was then sectioned, and carpal motion was recorded again. Carpal motions were compared with 37 normal wrists after sectioning of the scapholunate interosseous ligament and other ligaments to create a G4 wrist. RESULTS: Carpal motion of the 37 normal wrists after ligamentous sectioning was similar to motion of the 8 specimens noted to be G4. These wrists did not demonstrate subluxation of the scaphoid that may occur after ligament sectioning. After sectioning of the LTIL, there were significant changes in lunate and triquetral motion. CONCLUSIONS: These findings support the hypothesis that sectioning multiple ligaments in normal wrists to create scapholunate instability causes average motion comparable to that seen in G4 wrists. Ligamentous sectioning can cause a range of scaphoid instability. Lunotriquetral interosseous ligament sectioning in native G4 wrists caused greater changes in triquetral than scaphoid range of motion. CLINICAL RELEVANCE: Patients with arthroscopically determined G4 lesions have an incompetent SLIL and scapholunate instability but do not necessarily have scapholunate dissociation and subluxation. Cadaver studies that evaluate instability by sectioning specific intact wrist ligaments are similar to the G4 specimens and thus are a good approximation of naturally occurring wrist instability. The functionality of secondary stabilizers not seen arthroscopically may explain the differences in motion. Geissler IV wrists and ligament-sectioned wrists are points on the spectrum of carpal instability, which is determined by the extent of damage to multiple ligamentous structures.

Biomechanical Evaluation of Distal Radioulnar Joint Instability and Adams Procedure.

PURPOSE: Distal radioulnar joint (DRUJ) instability may occur after an injury, resulting in pain and reduced strength. When primary repair is not possible or initial fixation has failed, chronic instability may result, requiring a reconstructive procedure such as the Adams procedure. The first purpose of this study was to evaluate the role of the triangular fibrocartilage complex and various components of the interosseous membrane as they were sectioned. The second purpose was to evaluate the Adams procedure in stabilizing the forearm. METHODS: Eight fresh cadaver forearms were dynamically moved through an average range of 56.8° pronation to 54.8° supination and tested first with the forearm intact and then after sectioning each of the following structures: the dorsal (DRUL) and palmar radioulnar ligaments (PRUL), the distal interosseous membrane, and the central band. Finally, they were tested after reconstruction using the Adams procedure. During each forearm motion and provocative shuck, the motion of the radius and ulna were measured and the locations of the radial attachments of the DRUL, PRUL, and sigmoid notch and ulnar fovea were computed. RESULTS: Significant increases in the gap between the ulnar fovea and the attachment sites of the DRUL and PRUL were observed with incremental sectioning, most notably after sectioning of the central band. Reconstruction significantly reduced the gap at the DRUL and PRUL sites during dynamic motion. CONCLUSIONS: This study reinforces the concept that DRUJ stability depends on more than the radioulnar ligaments, ulnocarpal ligaments, and triangular fibrocartilage complex, but is also significantly affected by the distal and central interosseous membrane. Reconstruction reduces gapping. CLINICAL RELEVANCE: These results suggest that the Adams reconstruction is a reasonable option to address DRUJ instability but may be an incomplete solution in the setting of a ruptured interosseous ligament.

Capitate and Lunate Morphology in Normal Wrist Radiographs-A Pilot Study.

BACKGROUND: Morphology may provide the basis for the understanding of wrist mechanics. METHODS: We used classification systems based on cadaver dissection of lunate and capitate types to evaluate a normal database of 70 wrist radiographs in 35 subjects looking for associations between bone shapes. Kappa statistics and a log-linear mixed -effects model with a random intercept were used. RESULTS: There were 39 type-1, 31 type- 2 lunates, 50 spherical, 10 flat and 10 V-shaped capitates. There was a significant difference in lunate and capitate shape between the hands of the same individual p <0.001. This may be due to different loads on the dominant vs. nondominant hands in the same individual. CONCLUSION: Further study to better understand the development of radiographic parameters of the midcarpal joint may aid in our understanding of the morphology and mechanics of the wrist.

Changes in Scaphoid and Lunate Position and Loading at Two Wrist Pushup Positions.

BACKGROUND: Scapholunate interosseous ligament (SLIL) injury following a fall on an outstretched hand may lead to carpal instability and in some cases require long-term rehabilitation following repair. Rehabilitation, especially in athletes, may include pushups, but little is known as to what type of pushup may be safer. OBJECTIVE: To determine biomechanical differences between two pushup positions (neutral or extended). METHODS: Six fresh cadaver wrists with pre-existing SLIL damage were uni-axially loaded in neutral and extension in order to simulate two different pushup styles. The motions of the scaphoid and lunate in relation to the radius were measured. The dorsal, proximal, and volar insertion sites of the SLIL were identified and, using the collected kinematic data, gap distances were calculated for each site. RESULTS: Gap distance between the proximal SLIL insertion points was significantly greater in neutral than in extension. There was a trend that the dorsal and volar SLIL insertion points were also greater in neutral than in extension. After the wrist was extended 90o, the scaphoid extended 70.1o and the lunate extended 28.6o compared to their positions with the wrist in neutral. CONCLUSION: The larger gap distances between the scaphoid and lunate in neutral suggest that a neutral style pushup could put higher forces on a wrist with pre-existing SLIL damage and may thus hinder recovery for a person with a repaired SLIL. A pushup in extension, in these injured wrists, may be less detrimental.

Midcarpal Structure Effect on Force Distribution through the Radiocarpal Joint.

Background Wrist structure is complicated by distinct anatomical patterns. Previous studies defined radiographic wrist types based on lunate and capitate shape within the midcarpal joint. We hypothesized that these disparate structural patterns will transfer forces differently through the wrist. Objective This study aims to correlate force transferred to the distal radius and ulna with morphological measurements in cadaver arms. Methods Radiographs from 46 wrists, previously tested for force transfer between the radius and ulna, were examined. The percentage of compressive force through the distal ulna was determined by mounting load cells to the radius and ulna, while 22.2 Newton (N) tensile forces were individually applied to multiple tendons. Each wrist was tested in a neutral flexion-extension and radial-ulnar deviation position. Results Wrist type and lunate type were associated with percentage of force transfer through the ulna ( p = 0.002, p = 0.0003, respectively). Percentage of force transfer was correlated with capitate circumference ( p = 0.02, r = 0.34). Conclusions This study supports distinct force transfer between morphological wrist types. Clinical Relevance Understanding the mechanical significance of different structural variations in the wrist bones will improve our ability to understand wrist function and the distinctive development of wrist pathology. Level of Evidence This is a Level II study.

The use of a hydrogel implant in the repair of osteochondral defects of the knee: A biomechanical evaluation of restoration of native contact pressures in cadaver knees.

BACKGROUND: Osteochondral injuries have been treated by a variety of methods, each having its own drawbacks. The purpose of this study was to determine the biomechanical feasibility of using a hydrogel implant replacement for an osteochondral core defect. The hypothesis of this study was that the contact pressure of the native knee can be recreated with the use of a hydrogel implant. METHODS: Six cadaver knees were tested in a knee simulator while contact pressures were measured on the tibial plateau. Pressure data was collected in the intact knee, after coring of the condyle and after insertion of a hydrogel implant. Following 1000 gait cycles of fatigue testing, each knee was taken through axial loading indentation testing where the stiffness of the in situ implant was compared to the contralateral condyle. FINDINGS: While coring significantly reduced the peak pressure at the coring site from 1.8 MPa in the intact knee to 0.0 MPa after coring, implant insertion significantly increased it to 1.2 MPa. There was no significant difference in the peak pressures or the average pressures at the hole location between the intact knee and following implant insertion. After fatigue testing, no macroscopic loosening or implant damage was observed. Based on indentation testing, the stiffness of the medial condyle, 157 N/mm, was significantly less than the lateral condyle, 696 N/mm. INTERPRETATION: The insertion of the hydrogel implant was able to achieve restoration of contact pressures in the knee supporting the viability of hydrogel implants in the treatment of osteochondral lesions of the knee.

Distinct Wrist Patterns Founded on Measurements in Plain Radiographs.

Background In joints, structure dictates function and consequently pathology. Interpreting wrist structure is complicated by the existence of multiple joints and variability in bone shapes and anatomical patterns in the wrist. Previous studies evaluated lunate and capitate shape in the midcarpal joint, and two distinct patterns have been identified. Purpose Our purpose was to further characterize the two wrist patterns in normal wrist radiographs using measurements of joint contact and position. Our hypothesis was that we will find significant differences between the two distinct anatomical patterns. Patients and Methods A database of 172 normal adult wrist posteroanterior (PA) radiographs was evaluated for radial inclination, height, length, ulnar variance, volar tilt, radial-styloid-scaphoid distance, and lunate and capitate types. We measured and calculated percent of capitate facet that articulates with the lunate, scapholunate ligament, scaphoid, and trapezoid. These values were compared between the wrist types and whole population. Results Type-1 wrists (lunate type-1 and spherical proximal capitate) were positively associated with a longer facet between capitate and distal lunate ( p = 0.01), capitate and base of middle metacarpal ( p = 0.004), and shorter facet between the capitate and hamate ( p = 0.004). The odds ratio of having a type-1 wrist when the interface between the capitate and lunate measures >8.5 mm is 2.71 (confidence interval [CI] 1.07, 6.87) and when the line between the capitate and the base of middle metacarpal >9.5 mm is 3.5 (CI 1.38, 9.03). Conclusion We characterized the two-wrist patterns using intracarpal measurements. Translating these differences into three-dimensional contact areas may help in the understanding of biomechanical transfer of forces through the wrist. Level of Evidence This is a Level II, diagnostic study.

Carpal Pronation and Supination Changes in the Unstable Wrist.

Background Little is known about changes in scaphoid and lunate supination and pronation following scapholunate interosseous ligament (SLIL) injury. Information on these changes may help explain why some SLIL reconstructions have failed and help in the development of new techniques. Purpose To determine if following simulated SLIL injury there was an increase in scaphoid pronation and lunate supination and to determine if concurrently there was an increase in the extensor carpi ulnaris (ECU) force. Materials and Methods Scaphoid and lunate motion were measured before and after sectioning of the SLIL and two volar ligaments in 22 cadaver wrists, and before and after sectioning of the SLIL and two dorsal ligaments in 15 additional wrists. Each wrist was dynamically moved through wrist flexion/extension, radioulnar deviation, and a dart-throwing motion. Changes in the ECU force were recorded during each wrist motion. Results Scaphoid pronation and lunate supination significantly increased following ligamentous sectioning during each motion. There were significant differences in the amount of change in lunate motion, but not in scaphoid motion, between the two groups of sectioned ligaments. Greater percentage ECU force was required following ligamentous sectioning to achieve the same wrist motions. Conclusion Carpal supination/pronation changed with simulated damage to the scapholunate stabilizers. This may be associated with the required increases in the ECU force. Clinical Relevance In reconstructing the SLIL, one should be aware of the possible need to correct scaphoid pronation and lunate supination that occur following injury. This may be more of a concern when the dorsal stabilizers are injured.

Force Required to Maintain Reduction of a Preexisting Scapholunate Dissociation.

PURPOSE: To determine the force required to maintain reduction of Geissler grade 4 scapholunate dissociations during physiological wrist motions. METHODS: Fresh-frozen cadaveric wrists with Geissler grade 4 scapholunate dissociations were identified by arthroscopy. Following reduction, a load cell was attached to measure the force across the scapholunate joint at a neutral position and during 4 different wrist motions re-created using a servohydraulic wrist simulator, including a large flexion-extension motion (FEM), small and large dart-thrower's motions (DTMs), and a large DTM with ulnar offset. RESULTS: Five wrists with isolated preexisting scapholunate interosseous ligament (SLIL) tears were analyzed. The force required to maintain reduction was significantly greater in extension than in flexion during the large FEM and large DTM with ulnar offset. The forces were significantly larger in this study of preexisting SLIL dissociations compared with results from a prior study of acutely sectioned SLILs. In addition, forces to maintain reduction during DTMs were significantly less than forces during FEMs. Finally, a set of 3 wrists with preexisting scapholunate and lunotriquetral interosseous ligament (LTIL) tears were identified and had significantly decreased forces to maintain reduction at maximum extension and had decreased maximal forces during a cycle of motion compared with the wrists with isolated SLIL tears. CONCLUSIONS: The forces required to maintain reduction of a Geissler grade 4 wrist are higher than forces after acutely sectioned SLIL. The forces are greater in extension than in flexion and less during the DTM compared with the FEM. Wrists with both SLIL and LTIL tears required less force to maintain reduction than those with isolated SLIL tears. CLINICAL RELEVANCE: This study helps determine the strength of reconstruction required to maintain reduction of a Geissler grade 4 scapholunate dissociation.

Forces in the Distal Radius During a Pushup or Active Wrist Motions.

PURPOSE: To determine the 6 degrees of freedom forces and moments in the distal radius that occur during a pushup or other active wrist motions. METHODS: Eight fresh-frozen cadaveric wrists were moved through 6 physiological motions and held at 1 static pushup position while the force through the distal radius was measured with a 6 degrees of freedom load cell. Three levels of compressive force were applied at the pushup position. RESULTS: Active wrist motions caused axial forces up to 283 N and moments up to 0.7 N-m. Those motions with a smaller range had significantly smaller axial forces than the larger flexion-extension or dart-thrower's motions. With an 89-N pushup force applied, the average maximum axial force was 69 N, the radially directed force was 12 N, and the moment about the radioulnar axis was 2.3 N-m. Linear extrapolation of the forces to 100% body weight indicate that the axial force going through the distal radius would be 663 N, the radial force would be 147 N, and the moment about the radioulnar axis would be 18.6 N-m. CONCLUSIONS: Large distal radius forces and moments can occur during pushup and active wrist motions. There are not only large axial compressive forces but also nontrivial radially directed forces. CLINICAL RELEVANCE: This study may help surgeons and therapists better treat complicated distal radius fractures as well as provide for better comparisons of existing or new distal radius plates and constructs that are designed to treat these complicated loading patterns.

Effect of Push-Up Position on Wrist Joint Pressures in the Intact Wrist and Following Scapholunate Interosseous Ligament Sectioning.

PURPOSE: To determine the contact pressures between the scaphoid and lunate and the distal radius during 2 wrist push-up positions before and following scapholunate interosseous ligament (SLIL) sectioning. METHODS: Eight fresh cadaveric wrists were tested in a neutral flexion-extension (knuckle) push-up position and in an extended push-up position. Pressure measurements were acquired as each wrist was loaded with the wrist in extension and with the wrist in a neutral position. Data were acquired with the SLIL intact and following sectioning of its dorsal, volar, and proximal components. The wrist was disarticulated and a map drawn on each sensor to identify each joint fossa. RESULTS: A push-up performed with the wrist in extension caused a significantly greater peak pressure in the radioscaphoid fossa but not in the radiolunate fossa. Moving the wrist into extension caused a significant dorsal movement of the pressure centroid an average of 2.9 mm in the radiolunate fossa and an average of 5.7 mm in the radioscaphoid fossa. Sectioning the SLIL caused the centroid of pressure in the radioscaphoid fossa to significantly move an average 1.4 mm radially. CONCLUSIONS: A push-up with the wrist in extension causes a significant increase in the pressure in the radioscaphoid fossa but not in the radiolunate. This finding may help explain why degenerative arthritis first develops in the radioscaphoid fossa before involving the radiolunate fossa. As expected, gapping that occurs with SLIL injury was observed here as a radial translation of the scaphoid and not as a movement of the lunate. The new location of scaphoid contact may be an additional reason for the development of radioscaphoid arthritis occurring before radiolunate arthritis. CLINICAL RELEVANCE: This study provides a possible explanation as to why degenerative arthritis may first occur in the radioscaphoid fossa.

Biomechanical Analysis of Palmar Midcarpal Instability and Treatment by Partial Wrist Arthrodesis.

PURPOSE: To create a biomechanical model of palmar midcarpal instability by selective ligament sectioning and to analyze treatment by simulated partial wrist arthrodesis. METHODS: Nine fresh-frozen cadaver arms were moved through 3 servohydraulic actuated motions and 2 passive wrist mobilizations. The dorsal radiocarpal, triquetrohamate, scaphocapitate, and scaphotrapeziotrapezoid ligaments were sectioned to replicate palmar midcarpal instability. Kinematic data for the scaphoid, lunate, and triquetrum were recorded before and after ligament sectioning and again after simulated triquetrohamate arthrodesis (TqHA) and radiolunate arthrodesis (RLA). RESULTS: Following ligament sectioning, the model we created for palmar midcarpal instability was characterized by significant increases in (1) lunate angular velocity, (2) lunate flexion-extension, and (3) dorsal/volar motion of the capitate during dorsal/volar mobilizations. Simulated TqHA caused significantly more scaphoid flexion and less extension during the wrist radioulnar deviation motion. It also increased the amount of lunate and triquetral extension during wrist flexion-extension. Simulated RLA significantly reduced scaphoid flexion during both wrist radioulnar deviation and flexion-extension. CONCLUSIONS: Both simulated arthrodeses eliminate wrist clunking and may be of value in treating palmar midcarpal instability. However, simulated RLA reduces proximal row motion whereas simulated TqHA alters how the proximal row moves. Long-term clinical studies are needed to determine if these changes are detrimental. CLINICAL RELEVANCE: Palmar midcarpal instability is poorly understood, with most treatments based on pathomechanical assumptions. This study provides information that clinicians can use to design better treatment strategies for this unsolved condition.

Role of the Interosseous Membrane in Preventing Distal Radioulnar Gapping.

Background Damage to the interosseous membrane (IOM) can alter load transmission between the radius and ulna and decrease their axial stability. Less is known about the effect of IOM sectioning on the transverse stability between the radius and ulna. Purpose The purpose of this study was to quantify the radioulnar gapping at the distal radioulnar joint (DRUJ) during forearm rotation when the IOM was experimentally sectioned while maintaining the integrity of the distal radioulnar ligaments. Methods In 12 fresh-frozen cadaver forearms tested in a combined wrist-forearm simulator, the increase in gap between the radius and ulna, at the level of the DRUJ, was determined during cyclic forearm rotation following IOM sectioning. Results IOM sectioning caused a significant increase in dorsal gapping at the DRUJ by 2.1 mm in supination and 0.6 mm in pronation. It also caused an increase in palmar gapping by 1.3 mm in supination and 0.5 mm in pronation. Conclusion This experiment has shown that the IOM has an important role in stabilizing the DRUJ, especially in supination, and that IOM sectioning caused greater loads on the palmar and dorsal radioulnar ligaments. Since DRUJ instability is primarily treated by fixing the laxity at the dorsal radioulnar ligament (DRUL) and palmar radioulnar ligament (PRUL), untreated IOM damage could permit additional injury and instability to the radioulnar ligaments or their reconstruction. Clinical Relevance Reconstruction of a torn IOM should be considered in the presence of persistent DRUJ instability following DRUJ reconstruction.