Simulated scaphoid proximal pole fracture

Relationship between the fracture location and the kinematic pattern in scaphoid nonunion

PURPOSE

The purpose of this study was to obtain 3-dimensional and quantitative information regarding the pathological kinematics of the wrist with scaphoid nonunion using an in vivo and 3-dimensional motion analysis. We specifically tested the hypothesis that the fracture location is related to the kinematic pattern.

METHODS

We studied wrist kinematics during wrist flexion-extension motion and radioulnar deviation in 13 patients with scaphoid nonunion, using a markerless bone registration technique. Magnetic resonance images or computed tomography (CT) images of the wrist were acquired with the wrist in the neutral and 4 extreme positions of flexion, extension, radial deviation, and ulnar deviation. Three-dimensional animations were created of the carpal motions and interfragmentary motions between the distal and proximal fragments of the scaphoid. Based on the fracture location, accurate estimates of the relative positions and orientations of the carpal bones were analyzed.

RESULTS

There were 2 clear patterns of the interfragmentary motions of the scaphoid based on the fracture location. In the mobile type scaphoid nonunion (7 cases), the fracture was located distal to the apex of the scaphoid dorsal ridge (scaphoid apex), and the distal scaphoid was unstable relative to the proximal scaphoid. The distal fragment showed a "book-opening" motion from wrist flexion to extension. In the stable type scaphoid nonunion (6 cases), the fracture was located proximal to the scaphoid apex, and the interfragmentary motion was considerably less than with the distal type.

CONCLUSIONS

Carpal instability following scaphoid nonunion appears to be related to whether the fracture line passes distal or proximal to the scaphoid apex, where the dorsal scapholunate interosseous ligament and the dorsal intercarpal ligament attach. Three-dimensional imaging should be considered when assessing scaphoid nonunions to identify the exact location of the fracture. This may allow the clinician to choose the best treatment option.

Resection of the scaphoid distal pole for symptomatic scaphoid nonunion after failed previous surgical treatment

PURPOSE

To evaluate the results of resection of the scaphoid distal pole for symptomatic scaphoid nonunion after failed prior surgical treatment.

METHODS

Thirteen patients who were treated with resection of the scaphoid distal pole for persistent nonunion after previous surgical treatment were included in this study. The mean follow-up period was 5 years. Evaluation included measurement of wrist range of motion, assessment of pain, and evaluation of radiographic parameters. The Disabilities of the Arm, Shoulder, and Hand score was used to determine the functional outcome after the excision.

RESULTS

Before surgery all but 1 patient reported pain. After surgery 2 patients presented with mild pain during strenuous activity. Mean wrist flexion and extension increased significantly, by 23 degrees and 29 degrees, respectively. The postoperative Disabilities of the Arm, Shoulder, and Hand score was 25 +/- 19 points. There was a significant increase in the radiolunate angle, indicating dorsal intercalated segment instability deformity in 6 patients.

CONCLUSIONS

Scaphoid distal pole excision remains a valuable treatment option for patients for whom multiple attempts at union have failed previously and who have no associated complete scapholunate ligament tears.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic, Level IV.

Computer simulation of the upper extremities

The development of real-time, interactive, three-dimensional, computer graphic simulation of the musculoskeletal system with emphasis on the upper extremities is described. Developments in image analysis, scientific visualization, and interactive computer methods and the continuous improvement in knowledge of musculoskeletal function have combined to provide an exciting new tool for musculoskeletal research. Interactive simulation also has promising applications in medical education and clinical rehabilitation. Only those developments that fit the specific criteria of realism, real-time response, intuitive interaction, three-dimensional structures, and modeling flexibility are discussed. It is hoped that an improved understanding of this emerging tool and an appreciation for its potential applications will be gained.

Scaphoid nonunions: a 3-dimensional analysis of patterns of deformity

We tested the hypothesis that the fracture location of scaphoid nonunions relates to the fracture displacement, development of dorsal intercalated segment instability (DISI) deformity, and changes in the contact area of the bones in the radiocarpal joint. Eleven patients with scaphoid nonunions were examined with 3-dimensional computed tomography and a new method of proximity mapping. Two different patterns of displacement of scaphoid nonunions were demonstrated, 1 volar and 1 dorsal. All patients with a volar pattern scaphoid nonunion had a DISI deformity. Only a few of the patients with a dorsal pattern scaphoid nonunion, mostly in longstanding nonunions, had a DISI deformity. The fracture line was generally distal to the dorsal apex of the ridge of the scaphoid in the volar-type fractures and proximal in the dorsal displaced fractures. The proximity map of the distal fragment of the scaphoid on the radius in the volar type shifts radial compared with normal; in the distal type it shifts dorsal. Neither of the patterns showed any significant changes of the proximity map in the radiocarpal joint at the proximal scaphoid fragment and the lunate. Whether the fracture line passes distal or proximal to the dorsal apex of the ridge of the scaphoid appears to determine the likelihood of subsequent fracture displacement, DISI deformity, and contact area of the bones in the radiocarpal joint.

Proximal scaphoid costo-osteochondral replacement arthroplasty

Deficiency of the proximal pole of the scaphoid due to fracture or necrosis was treated by costo-osteochondral replacement arthroplasty using rib bone/cartilage autografts in 22 patients who were followed prospectively and assessed at a median 24 month follow-up (range, 12-72 months). Improvement of wrist function occurred in all patients with increased motion, improved grip strength and less pain. The average modified Green and O'Brien Wrist Function Score improved from 53 out of 100 preoperatively to 80 at the most recent review. All patients were graded fair or poor at initial review and all but three improved to good or excellent at the most recent assessment. Despite the absence of the scapholunate ligament, carpal alignment did not deteriorate in any patient and there were no graft non-unions or significant complications. In the short and medium term a costo-osteochondral autograft can satisfactorily restore mechanical integrity of the scaphoid proximal pole and maintain wrist motion while avoiding the potential complications of alternative replacement arthroplasty techniques.

Biomechanical changes of the wrist flexor and extensor tendons following loss of scaphoid integrity

Loss of integrity of the scaphoid may change the motion center of the entire carpus, and deformities from scaphoid fractures may alter the location of motor tendons of the wrist, thus altering their biomechanics. The goal of this study was to clarify biomechanical changes in these tendons following loss of scaphoid integrity. Excursions and moment arms of the principal flexor and extensor tendons of the wrist were investigated in seven cadaveric upper extremities in intact wrists after simulation of scaphoid waist fracture and after removal of the proximal scaphoid. Excursions of the flexor carpi radialis and ulnaris, extensor carpi radialis longus and brevis, and extensor carpi ulnaris tendons were measured with rotary potentiometers during wrist flexion-extension and radioulnar deviation. Simultaneously, wrist joint angulation was recorded. Moment arms of the tendons were derived from tendon excursions and joint motion. After scaphoid fracture, the moment arms of the flexor carpi radialis and extensor carpi ulnaris tendons increased significantly during wrist flexion-extension, whereas the moment arms of the extensor carpi radialis longus and brevis tendons decreased significantly. After proximal scaphoid excision, the moment arms of the extensor carpi radialis longus and brevis tendons again decreased significantly during wrist flexion-extension. The moment arms of the flexor carpi radialis and extensor carpi radialis brevis tendons increased significantly during radioulnar deviation, whereas those of the wrist motors on the ulnar side decreased. These findings indicate the importance of the integrity of the scaphoid in maintaining normal biomechanics of motor tendons of the wrist. An increase in the moment arm of the radial wrist flexor along with a decrease in moment arms of the radial extensors constitutes as etiology for persistent angulation of the scaphoid and the humpback deformity. In addition, disturbing the biomechanics of the wrist motor tendons predisposes the carpal joints to abnormal loading, potentially contributing to the development of carpal joint degeneration.

Biomechanics of the wrist

This article summarizes six years of studies with an experimental model, which uses a static positioning frame, pressure-sensitive film, and a microcomputer-based videodigitizing system, to characterize the biomechanics of the human cadaver wrist by measuring the contact areas and pressures in a variety of normal, simulated traumatic, and surgically treated conditions. The results of these studies have provided clinically relevant information about the normal anatomy and functional mechanics of the wrist, as well as guidelines for the treatment for a number of different fractures and ligament injuries. The results of studies of radial carpal instabilities and fractures have demonstrated an increased load in the areas where degenerative arthritis is seen in these clinical conditions, and have furnished a means by which the biomechanical efficacy of certain surgical treatments can be measured.

Limited arthrodesis for scaphoid nonunion

A new surgical technique is described for the treatment of longstanding scaphoid nonunions with established degenerative changes and carpal malalignment. The technique is based on natural history and biomechanical studies and includes the excision of the distal scaphoid fragment and a limited arthrodesis that incorporates the proximal pole of the scaphoid, the lunate, and the capitate. Six men were treated with the procedure for symptomatic chronic scaphoid nonunions, and all limited arthrodeses fused. Pain relief was good to excellent, and range of motion averaged 50% of the contralateral side.

Measurement of carpal bone geometry by computer analysis of three-dimensional CT images

The aim of this project was noninvasively to analyze and quantitate the geometry, load transfer characteristics, and spatial relationships of the carpal bones by using a new three-dimensional CT scan reconstruction technique. The determination of mechanical parameters such as distances between centroids and between bone surfaces, carpal alignment, volumes, surface areas, and contact areas can provide the basis for comparison between normal wrists and wrists with a variety of progressive instability patterns, types of fracture, pathologic and posttraumatic states, and different simulated surgical procedures. This new technology has demonstrated a volumetric accuracy of 94% and a linear accuracy of 97%. Simultaneous analysis of all articulating surfaces of multiple joints can be performed in cadavers and in patients because of the noninvasive nature of the imaging reconstruction technique. This new research offers much more information than has previously been available. It also promises direct application to the clinical setting and eliminates several limitations and questions that were inescapable with previous technology.

Load mechanics of the midcarpal joint

The midcarpal joint was studied in upper extremities of fresh cadavers with the use of a static positioning frame, pressure-sensitive film, and a microcomputer to analyze the contact prints. The contact area on the proximal side of the midcarpal joint was found to consist generally of four areas; the scaphoid-trapezium-trapezoid, the scaphoid-capitate, the lunate-capitate, and the triquetrum-hamate. The contact areas accounted for less than 40% of the available joint surface, even under loads of 118 pounds. The distribution of load through the midcarpal joint was scaphoid-trapezium-trapezoid 23%, scaphoid-capitate 28%, lunate-capitate 29%, and triquetrum-hamate 20%. The midcarpal joint, like the radiocarpal joint, appears to transmit load through distinct areas and through a relatively small portion of the available joint surface.