How Much Scaphoid Can be Safely Resected? A Biomechanical Analysis of the Effects of Distal Scaphoid Resection.

BACKGROUND: Resection of the distal pole of the scaphoid has been advocated as a simple alternative to other wrist salvage procedures for scaphoid nonunion advanced collapse and scaphotrapezio-trapezoid arthritis. However, the extent of scaphoid that may be resected without adversely affecting carpal kinematics has never been clearly defined. METHODS: Seven cadaveric upper extremities were tested in a custom motion wrist simulator. A 3-stage sequential sectioning of the distal scaphoid protocol was performed in 25% increments then cyclic active wrist flexion-extension and dart thrower's motion trials were recorded. RESULTS: The extent of distal scaphoid resection had no effect on overall wrist range of motion. The lunate assumed a more extended position following resection of the distal scaphoid compared to intact. At 25%, 50%, and 75% of distal scaphoid resection, the lunate extended to 13.32° ± 9.4°, 23.43° ± 7.5°, and 15.81° ± 16.9°, respectively. The capitate migrated proximally with 25% and 50% distal scaphoidectomy, and proximally and radially with 75% of the scaphoid resected. Resection of 75% of the scaphoid resulted in unstable wrist kinematics. CONCLUSIONS: Resection of up to 25% of the distal scaphoid did not significantly influence carpal kinematics and induced mild lunate extension deformity. Resection of 50% of the scaphoid induced further and potentially clinically significant lunate extension and dorsal intercalated segment instability. Further removal of 75% of the distal scaphoid induced capitate migration radially and unpredictable wrist kinematics. Consequently, removal of over 25% of the scaphoid should be avoided or supplemented with partial wrist fusion.

The Impact of Scaphoid Malunion on Carpal Motion: An In-Vitro Analysis.

Background: The clinical significance of scaphoid malunion is debated and its effect on wrist kinematics is poorly understood. Alterations to scaphoid morphology in other pathologies result in significant sequelae and arthrosis. By understanding the impact of scaphoid malunion on carpal kinematics, better insight can be garnered to inform models of wrist motion and clinical treatment of this injury. Methods: Seven cadaveric upper extremities underwent active wrist flexion and extension in a custom motion wrist simulator with scaphoid kinematics being captured with respect to the distal radius. A three-stage protocol of progressive simulated malunion severity was performed [intact, 10° malunion (Mal 10°), 20° malunion (Mal 20°)] with data analyzed from 45° wrist extension to 45° wrist flexion. Scaphoid malunions were modelled by creating successive volar wedge osteotomies and reducing the resultant scaphoid fragments with two 0.062" Kirshner wires. Motion of the scaphoid, lunate, trapezium-trapezoid was recorded using optical trackers. Results: Increasing scaphoid malunion severity did not significantly affect scaphoid or trapezium-trapezoid motion (p > 0.05); however, it did significantly alter lunate motion (p < 0.001). Increasing malunion severity resulted in progressive lunate extension across wrist motion (Intact - Mal 10°: mean difference (mean dif.) = 7.1° ± 1.6, p < 0.05; Intact - Mal 20°: mean dif. = 10.2° ± 2.0, p < 0.05). Conclusions: In this in-vitro model, increasing scaphoid malunion severity was associated with progressive extension of the lunate in all wrist positions. The clinical significance of this motion change is yet to be elucidated, but this model serves as a basis for understanding the kinematic consequences of scaphoid malunion deformities.

The Effect of Forearm Position on Wrist Joint Biomechanics.

PURPOSE: All active motion wrist joint simulators have been designed to simulate physiologic wrist motion; however, a main difference among them is the orientation of the forearm (horizontal or vertical with respect to gravity). Moreover, the effect of forearm orientation on experimental results has yet to be quantified, but it may be an important variable. Thus, the purpose of this study was to determine the effect of forearm orientation on wrist kinematics and contact mechanics. METHODS: Eight cadaveric upper limbs were cycled through a flexion-extension motion using an active motion wrist simulator. Motion trials were performed in 3 forearm orientations (gravity-neutral, gravity-flexion, and gravity-extension). A computed tomography-based joint congruency technique was used to examine radiocarpal joint contact and joint contact centroid translation in the 3 tested orientations. RESULTS: At full wrist extension and wrist flexion, radioscaphoid contact area was greatest in the gravity-extension orientation. Radiolunate contact area was similar among all 3 forearm orientations. The radioscaphoid contact centroid was consistent among the 3 tested positions with the wrist in neutral wrist position. In contrast, the radioscaphoid contact centroid translated radially in the gravity-neutral position relative to the gravity-flexion position in extreme extension. There were no differences in radiolunate centroid contact position in the 3 forearm orientations. CONCLUSIONS: This study demonstrates that forearm orientation affects contact mechanics and end-range carpal kinematics. Future biomechanical studies should report forearm orientation and discuss the implication of the forearm orientation used on the experimental results. CLINICAL RELEVANCE: This study provides evidence that the wrist joint is sensitive to forearm positions consistent with activities of daily living and rehabilitation protocols.

A Biomechanical Evaluation of the ECRL Tenodesis for Reconstruction of the Scapholunate Ligament.

PURPOSE: Reconstruction of the scapholunate ligament (SLL) in the setting of dynamic instability remains a surgical challenge, with lack of consensus on the best reconstructive procedure. Reconstruction of only the dorsal component may lead to volar gapping and abnormal wrist kinematics. This cadaveric active motion simulation study determined whether scapholunate (SL) motion, angulation, and contact are restored following open reconstruction using the extensor carpi radialis longus (ECRL) tenodesis, which reconstructs both the volar and the dorsal SLL components. METHODS: Seven fresh-frozen cadaveric upper limbs (mean age, 68 ± 10.1 years) underwent a 4-stage protocol of cyclic dart-throw motion and flexion-extension motion (utilizing an active wrist motion simulator that used tendon load/motion-controlled actuation. Scaphoid and lunate motion, relative scaphoid translation, SL angle, and dorsal-volar SL diastasis were measured with (1) wrist ligaments intact, (2) following complete sectioning of the SLL, and (3) following SL reconstruction using the ECRL tenodesis technique. RESULTS: Complete SLL sectioning resulted in a typical pattern of SL instability. Following the ECRL tenodesis, lunate extension was not corrected. Scaphoid flexion, however, was not significantly different from the native state in FEM but remained significantly flexed during dart-throw motion. Differential dorsal and volar gapping did not significantly improve following ECRL tenodesis (dorsal, 1.2-2.3 mm; volar, 1.1-1.7 mm). CONCLUSIONS: This biomechanical study demonstrates that the ECRL tenodesis did not fully restore native carpal kinematics, despite dorsal and volar SLL, and scaphotrapeziotrapezoid reconstruction. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.

The role of biceps loading and muscle activation on radial head stability in anterior Monteggia injuries: An in vitro biomechanical study.

INTRODUCTION: Little evidence-based information is available to direct the optimal rehabilitation of patients with anterior Monteggia injuries. PURPOSE OF THE STUDY: The aims of this biomechanical investigation were to (1) quantify the effect of biceps loading and (2) to compare the effect of simulated active and passive elbow flexion on radial head stability in anterior Monteggia injuries. STUDY DESIGN: In vitro biomechanical study. METHODS: Six cadaveric arms were mounted in an elbow motion simulator. The effect of biceps loading, simulated active and passive elbow flexion motions was examined with application of 0N, 20N, 40N, 60N, 80N, and 100N of load. Simulated active and passive elbow flexion motions were then performed with the forearm supinated. Radial head translation relative to the capitellum was measured using an optical tracking system. After testing the intact elbows, the proximal ulna was osteotomized and realigned using a custom jig to simulate an anatomical reduction. We then sequentially sectioned the anterior radiocapitellar joint capsule, annular ligament, quadrate ligament, and the proximal and middle interosseous membrane to simulate soft tissue injuries commonly associated with anterior Monteggia fractures. RESULTS: Greater magnitudes of biceps loading significantly increased anterior radial head translation. However, there was no significant difference in radial head translation between simulated active and passive elbow flexion except in the final stage of soft tissue sectioning. There was a significant increase in anterior radial head translation with progressive injury states with both isometric biceps loading and simulated active and passive motion. CONCLUSIONS: Our results demonstrate that anatomic reduction of the ulna may not be sufficient to restore radial head alignment in anterior Monteggia injuries with a greater magnitude of soft tissue injury. In cases with significant soft tissue injury, the elbow should be immobilized in a flexed and supinated position to allow relaxation of the biceps and avoid movement of the elbow in the early postoperative period.

The Impact of Scaphoid Malunion on Radioscaphoid Joint Contact: A Computational Analysis.

PURPOSE: The clinical relevance of scaphoid malunion is controversial because the biomechanical sequelae remain poorly understood. In this computational study, the effect of increasing scaphoid malunion on radioscaphoid joint contact was assessed. METHODS: Six computational wrist models of active wrist flexion-extension were used to examine 6 scaphoid malunions of varying severities. The malunions were computationally created using 3-dimensional imaging software. Each scaphoid was shortened at the waist by 2 mm to simulate fracture comminution and the distal pole was angulated volarly from 15° to 55° in 10° intervals to create a total of 6 scaphoid malunion models per specimen. Each malunion model was then assessed at 3 wrist positions: neutral, 40° flexion, and 40° extension. The radioscaphoid contact area, as well as the contact centroid, was calculated. RESULTS: There was a statistically significant association between malunion severity and a wider area of contact at the radiocar: pal joint. The centroid of this radioscaphoid contact area also moved in an ulnar direction for all tested wrist positions. In the extended wrist position, the centroid also moved volarly. CONCLUSIONS: In this computational model, the amount of radioscaphoid joint contact was significantly increased with progressive scaphoid malunion severity. Furthermore, the position of the contact centroid had a significant change in position in the ulnar direction in all wrist positions with increasing scaphoid malunion. The clinical importance of this contact is yet to be elucidated, but this computational model serves as a basis for understanding the biomechanical consequences of scaphoid humpback deformities. CLINICAL RELEVANCE: This study provides a computational analysis examining changes at the radioscaphoid joint that may occur within the wrist with the scaphoid malunited.

Effect of ulnar angulation and soft tissue sectioning on radial head stability in anterior Monteggia injuries: an in vitro biomechanical study.

BACKGROUND: Radial head instability continues to be a challenge in the management of anterior Monteggia injuries; however, there is a paucity of literature on the factors that contribute to this instability. The aim of this biomechanical investigation was to examine the effects of ulnar angulation and soft tissue insufficiency on radial head stability in anterior Monteggia injuries. METHODS: Six cadaveric arms were mounted in an elbow motion simulator. Radial head translation was measured during simulated active elbow flexion with the forearm supinated. After testing the elbows in the intact state, the ulna was osteotomized and tested at 0°, 10°, 20°, and 30° of extension angulation. To examine the effect of soft tissue insufficiency, the anterior radiocapitellar joint capsule, annular ligament, quadrate ligament, and the proximal and middle interosseous membrane (IOM) were sequentially sectioned. RESULTS: There was a significant increase in anterior radial head translation with greater ulnar extension angulation. Sequential soft tissue sectioning also significantly increased anterior radial head translation. There was no increase in radial head translation with isolated sectioning of the anterior radiocapitellar joint capsule. Additional sectioning of the annular ligament and quadrate ligament slightly increased anterior radial head translation but did not reach statistical significance. Subsequent sectioning of the proximal and middle IOM resulted in significant increases in anterior radial head translation. CONCLUSION: Our study demonstrates that progressive ulnar extension angulation results in an incremental increase in anterior radial head translation in anterior Monteggia injuries. Moreover, increasing magnitudes of soft tissue disruption result in greater anterior radial head instability.

An In Vitro Study to Determine the Effect of Ulnar Shortening on Distal Forearm Loading During Wrist and Forearm Motion: Implications in the Treatment of Ulnocarpal Impaction.

PURPOSE: To evaluate the effect of ulnar shortening on distal forearm loading following simulated dynamic motion. METHODS: Ulnar shortening was simulated using a custom-built adjustable implant to simulate up to 4 mm of ulnar shortening (-4 mm) in 9 cadaveric extremities. Load cells were placed in the distal ulna and radius to quantify axial loading. Using a wrist and forearm motion simulator, absolute and percentage loads were measured during dynamic flexion, ulnar deviation (UD), flexion dart throw (DT), and pronation. RESULTS: There was a significant decrease in absolute and percentage distal ulnar loads at each interval of ulnar shortening during flexion, UD, DT, and pronation. The distal ulna bore no compressive loads, and in fact, tensile loads were measured in the ulna at 2 mm of ulnar shortening during DT and pronation, at 3 mm during flexion, and at 4 mm during UD. CONCLUSIONS: A progressive decrease in distal ulnar loads with generation of tensile loads was observed with sequential ulnar shortening. CLINICAL RELEVANCE: Ulnar shortening greater than 2 mm can result in tensile loading in the distal ulna. When managing ulnar impaction syndrome, excessive shortening may not be required to provide relief of symptoms.

Effect of Radial Lengthening on Distal Forearm Loading Following Simulated In Vitro Radial Shortening During Simulated Dynamic Wrist Motion.

PURPOSE: To evaluate the effect of radial length change on distal forearm loading during simulated dynamic wrist motion. METHODS: A custom-built adjustable radial implant was used to simulate up to 4 mm of distal radius shortening (-4 mm) and 3 mm of lengthening (+3 mm). Load cells were placed in the distal radius and ulna in cadavers to measure their respective axial loads. The specimens were mounted on a wrist motion simulator that produced active wrist motion via tendon actuation. To simulate radial lengthening osteotomy following radial shortening from malunion, the radius was sequentially lengthened by 1-mm intervals from -4 mm to +3 mm. Radial and ulnar loads were measured during simulated wrist flexion, ulnar deviation (UD), and flexion dart throw (DT) at each interval of radial lengthening up to +3 mm. RESULTS: During wrist flexion and UD, for each millimeter of radial lengthening from -4 mm to the native length, there was a significant increase in distal radial loads. No significant change in radial load was observed beyond the native length during flexion and UD. There was no change in distal radial loads during DT for each interval of radial lengthening from -4 mm to +3 mm. A sequential decrease in ulnar loads was observed as the radius was lengthened from -4 mm to +3 mm for all wrist motions evaluated. CONCLUSIONS: Radial lengthening beyond the native length was not detrimental to radial loading and further reduced distal ulnar loading; achieving at least native ulnar variance seems to be appropriate to restore normal biomechanical loading based on this in vitro study. CLINICAL RELEVANCE: Lengthening of the radius beyond native variance in the setting of ulnar impaction syndrome, distal radius malunion, or distal radioulnar instability may not result in excessive loading of the distal radius and further reduces loading on the distal ulna. Surgeons should obtain contralateral wrist x-rays to serve as a template when performing distal radius osteotomies.

Carpal Kinematics following Sequential Scapholunate Ligament Sectioning.

Background The scapholunate ligament (SLL) is the most commonly injured intercarpal ligament of the wrist. It is the primary stabilizer of the scapholunate (SL) joint, but the scaphotrapeziotrapezoid (STT) and radioscaphocapitate (RSC) ligaments may also contribute to SL stability. The contributions of SL joint stabilizers have been reported previously; however, this study aims to examine their contributions to SL stability using a different methodology than previous studies. Purpose The purpose of this in vitro biomechanical study was to quantify changes in SL kinematics during wrist flexion and extension following a previously untested sequential sectioning series of the SL ligament and secondary stabilizers. Methods Eight cadaveric upper extremities underwent active wrist flexion and extension in a custom motion wrist simulator. SL kinematics were captured with respect to the distal radius. A five-stage sequential sectioning protocol was performed, with data analyzed from 45-degree wrist flexion to 45-degree wrist extension. Results Wrist flexion and extension caused the lunate to adopt a more extended posture following sectioning of the SLL and secondary stabilizers compared with the intact state ( p < 0.009). The isolated disruption to the dorsal portion of the SLL did not result in significant change in lunate kinematics compared with the intact state ( p > 0.05). Scaphoid kinematics were altered in wrist flexion following sequential sectioning ( p = 0.013). Additionally, disruption of the primary and secondary stabilizers caused significant change to SL motion in both wrist flexion and wrist extension ( p < 0.03). Conclusions The SLL is the primary stabilizer of the SL articulation, with the STT and RSC ligaments playing secondary stabilization roles. Clinical Relevance Understanding the role primary and secondary SL joint stabilizers may assist in the development of more effective treatment strategies and patient outcomes following SLL injuries.

The Effect of Dorsally Angulated Distal Radius Deformities on Carpal Kinematics: An In Vitro Biomechanical Study.

PURPOSE: The purpose was to quantify the effect of distal radius dorsal angulation (DA) on carpal kinematics and the relative roles of the radiocarpal and midcarpal joints during wrist motion. METHODS: Six cadaveric specimens (69 ± 17 y) were mounted at 90° elbow flexion in a custom wrist motion simulator. The wrist was guided through planar passive flexion and extension motion trials (∼ 5°/s). A custom modular distal radius implant was used to simulate native alignment and 3 distal radius DA deformities (10°, 20°, 30°). An optical tracking system captured carpal bone motion, from which radiocarpal and midcarpal joint motion was determined. RESULTS: The radiocarpal joint made a greater contribution to wrist motion than the midcarpal joint in flexion, and the midcarpal joint made a greater contribution to motion than the radiocarpal joint in wrist extension. Increasing DA caused the radiocarpal joint contribution to increase throughout the motion arc, with the effect being more pronounced in wrist flexion. Conversely, as DA increased, the midcarpal joint contributed less rotation to the total wrist motion and its overall motion arc decreased; the magnitude of effect was greater in wrist extension. Dorsal angulation resulted in increased lunate flexion with respect to the distal radius. CONCLUSIONS: Our findings agree with current literature that suggests that, in an uninjured wrist, the radiocarpal joint predominates flexion, and the midcarpal joint predominates extension. In addition, the radiocarpal joint has an amplified contribution in wrist flexion with greater DA malunion. CLINICAL RELEVANCE: The altered contributions of the radiocarpal and midcarpal joints may contribute to pain, stiffness, and the development of arthritis, which is commonly seen at the radiocarpal joint after malunion of the distal radius.

Biomechanical Evaluation of Carpal Kinematics during Simulated Wrist Motion.

Background Flexion and extension of the wrist is achieved primarily at the radiocarpal and midcarpal joints. Carpal kinematics have been investigated, although there remains no consensus regarding the relative contribution of each bone to wrist motion. Purpose To determine the kinematics of the scaphoid, lunate, and capitate during unconstrained simulated wrist flexion/extension and to examine the effect of motion direction on the contribution of each bone. Materials and Methods Seven cadaveric upper extremities were tested in a passive wrist simulator with 10N tone loads applied to the wrist flexors/extensors. Scaphoid, lunate, and capitate kinematics were captured using optical tracking and analyzed with respect to the radius. Results Scaphoid and lunate motion correlated linearly with wrist motion (R2 = 0.99, 0.97). In extension, the scaphoid and lunate extended 83 ± 19% and 37 ± 18% relative to total wrist extension (p = 0.03, 0.001), respectively. In flexion, the scaphoid and lunate flexed 95 ± 20% and 70 ± 12% relative to total wrist flexion (p = 1.0,0.01) , respectively. The lunate rotated 46 ± 25% less than the capitate and 35 ± 31% less than the scaphoid. The intercarpal motion between the scaphoid and lunate was 25 ± 17% of wrist flexion. Conclusion The scaphoid, lunate, and capitate move synergistically throughout planar wrist motion. The scaphoid and lunate contributed at a greater degree during flexion, suggesting that the radiocarpal joint plays a more critical role in wrist flexion. Clinical Relevance The large magnitude of differential rotation between the scaphoid and lunate may be responsible for the high incidence of scapholunate ligament injuries. An understanding of normal carpal kinematics may assist in positioning carpal bones during partial wrist fusions and in developing more durable wrist arthroplasty designs.