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Comparison of Smooth-Stemmed Bipolar with In-Growth Monopolar Radial Head Implants
T. Sean Lynch, MD1; John C. Berschback, MD1; David M. Kalainov, MD1; Robert W. Wysocki, MD2; Mark S. Cohen, MD2; Bradley R. Merk, MD1;
1Northwestern University, 2Midwest Orthopeadics, Rush University Medical Center

Introduction: There is a paucity of comparative data to guide implant choice for radial head replacements. The purpose of our study was to evaluate the clinical and radiographic results between patients who received a smooth-stemmed bipolar radial head implant and patients who received an in-growth monopolar radial head prosthesis. Causes of periprosthetic osteolysis in both groups were investigated.

Methods: We retrospectively reviewed 27 consecutive adult patients who underwent radial head implant arthroplasty with either a smooth-stemmed bipolar implant (14 cases, Group 1) or an in-growth monopolar implant (13 cases, Group 2). All patients returned for follow-up evaluation at an average of 33 months after surgery (range, 18 to 57 months). Outcome assessments included joint motion, elbow stability, grip strength, a visual analog scale (VAS) for pain, the Mayo Elbow Performance Index (MEPI), and the Disability of Arm, Shoulder and Hand (DASH) questionnaire. Radiographs were reviewed for joint congruence, heterotopic ossification, periprosthetic osteolysis, and degenerative arthritis. Three patients from the bipolar group and 4 patients from the monopolar group with periprosthetic osteolysis were tested for serum metal ion levels (cobalt, chromium, titanium) and inflammatory markers to determine if metal-degradation products and/or inflammation were associated with osteolysis

Results: Elbow flexion was greater in Group 1, whereas forearm pronation was greater in Group 2. There were no significant differences between the two implant groups for grip strength, elbow extension, forearm supination, or VAS pain, MEPI, or DASH scores. Bony overgrowth occurred in both groups of patients around the radial neck and exposed portion of the implant; however, there was a trend for excessive bone formation in Group 1. Periprosthetic osteolysis was also seen in both groups of patients, but was more pronounced with occasional ballooning of the cortex in Group 2. The serologic markers of inflammation were normal in the patients tested, and the metal ion levels did not exceed the levels reported for individuals with a well-functioning unilateral total hip arthroplasty.

Conclusion: Clinical outcomes at short- to mid-term follow-up were similar between patients treated with either a smooth-stemmed bipolar or an in-growth monopolar radial head implant. Periprosthetic bone overgrowth occurred more commonly in association with the smooth-stemmed bipolar implant, while osteolytic changes were more pronounced in association with the in-growth monopolar implant. We suspect that ballooning osteolysis seen in some of the in-growth implant cases was due to excessive motion of the roughened stem surface with failure of osseointegration.

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