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Load-Sharing at the Wrist Following Radial Head Replacement with a Metal ImplantA Cadaveric Study
Keith L. Markolf, PhD1; Samir G. Tejwani, MD1; Geoffery O'Neil, BS1; Prosper Benhaim, MD1
1 Biomechanics Research Section, Department of Orthopaedic Surgery, David Geffen School of Medicine at University of California at Los Angeles, Rehabilitation Building, 1000 Veteran Avenue, Room 21-67, Los Angeles, CA 90095. E-mail address for K.L. Markolf: kmarkolf@mednet.ucla.edu
View Disclosures and Other Information
In support of their research or preparation of this manuscript, one or more of the authors received Grant R01 AR43735 from the National Institute of Arthritis and Musculoskeletal and Skin Diseases/National Institutes of Health. None of the authors 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, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at the Biomechanics Research Section, Department of Orthopaedic Surgery, David Geffen School of Medicine at University of California at Los Angeles, Los Angeles, California

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2004 May 01;86(5):1023-1030
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Background: Surgical excision of the radial head is frequently required after a comminuted fracture of the radial head. The outcome of this procedure is often unpredictable, with some patients experiencing ulna-sided pain in the wrist secondary to proximal migration of the radius. Insertion of a radial head prosthesis could prevent proximal radial migration and restore normal load-sharing at the wrist. The thickness of the radial head implant is an important variable in restoring anatomical radial length; however, the effects of varying the length of implants that were used to reconstruct the radius on load-sharing at the wrist have not been studied biomechanically, to our knowledge.

Methods: A miniature load cell was attached to fifteen fresh-frozen cadaveric forearms to record force in the distal part of the ulna as the wrist was axially loaded to 134 N of compression force. Proximal displacement of the radius relative to the capitellum was also recorded. Loading tests on intact forearms were performed with the elbow in valgus and varus alignment and with three positions of wrist rotation (neutral, 45° of pronation, and 45° of supination). Loading tests were then repeated, with the same positions of varus and valgus elbow alignment and wrist rotation as had been used in the tests of the intact forearm, after radial head excision and subsequent insertion of metal radial head implants that restored anatomical length, implants that produced a radial length that was longer than the anatomical length, and implants that produced a radial length that was shorter than the anatomical length. Testing of these different implant thicknesses was repeated after sectioning of the interosseous membrane.

Results: The mean distal ulnar forces and mean proximal radial displacements following insertion of an implant that restored anatomical length were not significantly different from the corresponding values for the intact forearm. At neutral wrist rotation, replacing that implant with an implant that increased the radial length by 4 mm (after sectioning of the interosseous membrane) decreased the mean distal ulnar force from 13.4% to 3.3% of the applied wrist force with the elbow in valgus alignment and from 29.1% to 8.6% with the elbow in varus alignment. Replacing the implant that restored anatomical length with one that decreased the length by 4 mm (after sectioning of the interosseous membrane) significantly increased the mean distal ulnar force from 13.4% of the applied wrist load to 33.3% with the elbow in valgus alignment and from 29.1% to 51.6% with it in varus alignment. The mean distal ulnar forces were not significantly affected by the position of wrist rotation when the elbow was in valgus alignment. However, when the elbow was in varus alignment, the mean distal ulnar forces associated with all reconstructed radial lengths were significantly higher when the wrist was placed in 45° of supination.

Conclusions: In this cadaveric model, insertion of a metal implant maintained distal ulnar forces at normal levels, at all three positions of wrist rotation, when the radius had been restored to its original anatomical length. Distal ulnar forces and proximal radial displacements were significantly affected by the reconstructed length of the radius.

Clinical Relevance: Radial head implants are utilized to prevent proximal migration of the radius as the wrist is loaded; this is especially important when the interosseous membrane has been ruptured and thus cannot help to limit radial displacement. At the time of surgery, comminution and displacement of a radial head fracture may make estimation of the original radial length difficult. Our results demonstrate that, in terms of distal ulnar loading, it is preferable to insert an implant that is too thick rather than too thin.

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    Accreditation Statement
    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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    Keith L Markolf
    Posted on May 28, 2004
    Dr. Markolf responds:
    University of California at Los Angeles

    To the Editor:

    Clearly, one should aim to restore the radius to its original length if possible, and our results support this. Clinically, a surgeon may utilize radiographic measurements of the contralateral radial length as a guide or may examine the relative radius/ulna length at the time of surgery with different implant sizes until an appropriate length and DRUJ congruency are achieved.

    This biomechanical study focused on the effects on load transmitted through the distal ulna when changing the radial implant thickness. We found that when the reconstructed radius was too short, distal ulnar force was increased, thereby negating the desired objective of the operation. We concluded that if it is not possible to measure or estimate the original radial length, it is better from a biomechanical perspective to insert an implant that is slightly too thick, rather than too thin, although certainly within a relatively narrow range of radial head implant thicknesses to avoid "overstuffing” of the joint.

    By stating that the surgeon should insert the thickest implant possible, we did not mean to imply that the joint should always be "overstuffed". Forcing an implant into a space that is too small could increase contact stresses at the cartilage interface and possibly produce wear. Rather, we meant to state that if the choice of trial implants comes down to one that is slightly too thick vs. slightly too thin, the thicker one should be selected. In the discussion section of our manuscript, we noted that an implant that was thicker than normal would increase proximal radial load, which in turn could have adverse clinical consequences such as "increased cartilage wear at the interface, implant loosening, elbow arthritis, pain, and loss of motion".

    The anecdotal case report by Van Reit et al described extreme wear of the cartilage and underlying bone in a patient that had undergone radial head replacement forty-four months after initial resection of the radial head. Disuse osteoporosis was cited as a major contributing factor in this highly unusual case of extreme wear of cartilage and bone. "Overstuffing" of the radiohumeral joint was also mentioned as a possibility, although there were no indications that the authors attempted to estimate the original radial length by radiographic measurements of the contralateral forearm.

    The incidence and severity of capitellar wear in conjunction with metallic radial head replacements is an interesting topic, which deserves further clinical study.

    David Ring
    Posted on May 13, 2004
    Do Not Use the Largest Radial Head Possible
    Massachusetts General Hospital

    To the Editor:

    The conclusion of Markolf and colleagues to "insert the thickest radial head implant possible" is, in my view, incorrect. Metallic radial head implants are currently in wide use and are effective in stabilizing the elbow and forearm after fracture- dislocation. However, the main drawbacks of metallic radial head implants include capitellar wear, lateral gapping of the ulnohumeral joint, and synovitis (see the case report in the same issue of the Journal; Van Riet and colleageus). These are usually only a problem when the radial head implant is too large.

    The issue of whether radial head resection affects function of the wrist or distal radioulnar joint in the absence of injury to the interosseous ligament has been a traditional source of interest, but not a major source of clinical problems. So-called "over-stuffing" of the radiocapitellar joint is a recognized problem and the major pitfall of the use of a metallic radial head prosthesis.

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