The Journal of Bone and Joint Surgery (American). 2008;90:1979-1987.
doi:10.2106/JBJS.G.01015
© 2008 The Journal of Bone and Joint Surgery, Inc.
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Muscle Moment Arms in the First Dorsal Extensor Compartment After Radial Malunion

A Cadaver Study

Elly S. LaRoque, MD1, Wendy M. Murray, PhD2, Sarah Langley, MD3, Sanaz Hariri, MD4, Benjamin Parker Levine, MD5 and Amy L. Ladd, MD6

1 Post Street Orthopaedics and Sports Medicine, 2299 Post Street, Suite #103, San Francisco, CA 94115
2 Departments of Biomedical Engineering and PM&R, Northwestern University, VA Hines, 345 East Superior Street, Suite 1408, Chicago, IL 60611
3 Stanford University School of Medicine, 300 Pasteur Drive, Stanford, CA 94305
4 Harvard Combined Orthopaedic Residency Program, 55 Fruit Street, WHT 535, Boston, MA 02114
5 150 Presidential Way, Woburn, MA 01801
6 Robert A. Chase Hand and Upper Limb Center, Department of Orthopaedic Surgery, Stanford University, 770 Welch Road, Suite 400, Palo Alto, CA 94304. E-mail address: alad{at}stanford.edu
Investigation performed at Stanford University School of Medicine, Stanford, California, and the Palo Alto Veterans Affairs Health Care System, Palo Alto, California

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants of less than $10,000 from the Iris F. Litt, M.D. Fund from the Institute for Research on Women and Gender, Stanford University. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Background: Functional loss is a common complication of the fractured distal part of the radius. The purpose of the present study was to determine if the moment arms of the first dorsal extensor compartment are altered by distal radial fracture malunion. We hypothesized that the moment arms of the abductor pollicis longus and extensor pollicis brevis are significantly affected by dorsal angulation, radial inclination, and radial shortening, the most common deformities accompanying distal radial malunion.

Methods: Moment arms of the extensor pollicis brevis and abductor pollicis longus were estimated in twelve cadaver wrists with use of the tendon-displacement method, which involves calculating the moment arm as the derivative of tendon displacement with respect to joint angle. Tendon displacement was quantified in different wrist postures before and after a closing-wedge osteotomy simulating a complex malunion of an extra-articular radial fracture.

Results: The simulated distal radial malunion resulted in a decrease in the wrist flexion moment arm for both the extensor pollicis brevis (p = 0.0003) and the abductor pollicis longus (p < 0.0001). The wrist flexion moment arms for the extensor pollicis brevis and abductor pollicis longus decreased by a mean (and standard deviation) of 114% ± 75% and 77% ± 50%, respectively, after the osteotomy. The wrist radial deviation moment arms for the extensor pollicis brevis and abductor pollicis longus increased by 16% ± 26% (p = 0.071) and 28% ± 44% (p = 0.043), respectively, after the osteotomy. Radiographs of the wrist that were made before and after the osteotomy indicated that radial tilt changed from 11.1° of volar angulation to 14.8° of dorsal angulation, radial inclination decreased from 21.8° to 7.7°, and radial height decreased from 11.6 to 4.4 mm.

Conclusions: Distal radial malunion alters the mechanical advantage of the muscles in the first dorsal extensor compartment.

Clinical Relevance: The present study reinforces current treatment methods that restore architecture to improve function following a distal radial fracture. In particular, for levels of deformity similar to those tested here, the results of the present study support corrective osteotomy to improve the biomechanics of the thumb tendons of the first dorsal compartment.


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