Shawn W. O'Driscoll

Radiocapitellar joint stability with bipolar versus monopolar radial head prostheses

BACKGROUND Bipolar and monopolar designs are both available for replacement of the radial head. Few data exist comparing the biomechanical characteristics of these 2 quite different prostheses. This study evaluated the relative contribution to radiocapitellar stability by concavity compression with these 2 types of radial head prostheses. METHODS The study used 12 fresh frozen elbow cadavers. The capitellum of the distal humerus and 3 different conditions of radial head (native, monopolar and bipolar) were tested for radiocapitellar joint stability. RESULTS The monopolar metallic head and the native radial head behaved similarly regarding resistance to subluxation. The bipolar head behaved in an entirely opposite manner than the native and monopolar head and actually acted to facilitate subluxation. CONCLUSIONS Mobility of radial head components, such as in the bipolar radial head, has a compromising effect on the concavity compression stability of the radiocapitellar joint. A monopolar implant is more effective in stabilizing the radiocapitellar joint than a bipolar radial head prosthesis.

Grip strength testing using the BTE work simulator and the jamar dynamometer: A comparative study*

This study compared the results obtained from testing grip strength using the hydraulic Jamar dynamometer and the BTE Work Simulator. The grip strength of 14 adults was assessed with the Jamar at 90 degrees elbow flexion, Jamar at 45 degrees elbow flexion, and the BTE using the manufacturers recommended position. Greater strength measurements were generated with the elbow at 90 degrees rather than 45 degrees on the Jamar. The Jamar measurements at 90 degrees of elbow flexion were not significantly different from the BTE work simulator values. Intraclass correlation coefficients between the Jamar at 90 degrees and the BTE work simulator values were excellent. This study demonstrated that the results obtained from the BTE work simulator and the Jamar dynamometer are comparable. These findings validate the use of the BTE work simulator for grip strength testing.

Stem diameter and micromotion of press fit radial head prosthesis: A biomechanical study

BACKGROUND Aseptic loosening of the stem, ranging from mild periprosthetic lucency to symptomatic loosening leading to implant removal, has been reported in press fit radial head prostheses. HYPOTHESIS The purpose of this study was to determine the effect of the stem diameter and insertion force on initial stability with a press fit radial head prosthesis designed for bone ingrowth. MATERIALS AND METHODS Cadaveric radii were implanted with radial head prostheses of increasing stem diameter. The insertion forces for each rasp and stem were measured. Micromotion of the stem at the isthmus of the canal and stem tip were measured under circumstances simulating eccentric loads. RESULTS Insertion forces for all submaximum-sized rasps were similar. However, the insertion force for the maximum-sized rasp was approximately twice as large, and the insertion force for the oversized rasp was twice as large again, potentially indicating that the insertion force may be useful as a guide for determining appropriate stem size. Micromotions of the maximum diameter stem (isthmus, 41 microm; tip, 64 microm) were near the threshold for bone ingrowth, whereas the micromotions of the submaximum stem (isthmus, 253 microm; tip, 394 microm) were above this threshold. DISCUSSION The maximum diameter stem achieved greater stability (minimum micromotion) compared with the submaximum diameter stem. CONCLUSION The best fixation strength in the press fit radial head prosthesis was achieved by maximum sizing in the neck canal.

Radial head prosthesis micromotion characteristics: Partial versus fully grit-blasted stems

BACKGROUND Compared to fully textured prosthetic stems, partial texturing lessens bone loss due to stress shielding and makes removal easier. However, initial press-fit stability is necessary for bone ingrowth. HYPOTHESIS There is no significant difference in the initial stability of radial head prostheses that are partially grit-blasted compared to those that are fully grit-blasted. MATERIALS AND METHODS Cadaveric radii were implanted with partial or fully grit-blasted radial head prostheses. Micromotion of the stem at the isthmus of the canal and stem tip were measured under circumstances simulating eccentric loads. RESULTS Micromotion was not significantly different in the fully grit-blasted stems (isthmus, 11 ± 1 μm; tip, 21 ± 2 μm) and partially grit-blasted stems (isthmus, 13 ± 2 μm; tip, 25 ± 2 μm) (P = 0.4). The direction of loading had no effect on micromotion characteristics in either the fully or partially grit-blasted stems (P = .07). DISCUSSION Micromotion is comparable in partially and fully grit-blasted radial head prosthetic stems. For both stem surfaces, micromotion was well within the range that is conducive for bone ingrowth. A partially textured stem might have less bone loss due to stress shielding, making it easier to remove without destroying bone. CONCLUSION The initial stability of a radial head stem that is partially grit-blasted only at the proximal end is comparable to that of a radial head stem that is grit-blasted along its entire length.

Editorial Commentary: It's the Fracture, Not the Fragment, That Causes the Pain: Posteromedial Elbow Impingement in Baseball Players

Posterior impingement pain in baseball players is typically due to a fractured olecranon osteophyte. Computed tomographic (CT) scanning is the imaging of choice when evaluating posterior or posteromedial elbow pain in athletes who have subjected the elbow to excessive stresses such as occur in overhead throwing and weightlifting. Sometimes the fracture fragment can be so small that it might not be detected on plain radiographs, magnetic resonance imaging, or even at the time of arthroscopy. Symptomatic fractured olecranon osteophytes can be diagnosed clinically with the extension impingement test or arm bar test. Posteromedial osteophytes are often part of a generalized hypertrophic osteoarthritic response to overload and/or wear in the joint, which are best seen on CT scans.