gms | German Medical Science

Objectives: Elbow injuries are a very common issue. In case of fractures and degenerative changes, a radial head prosthesis (RHP) is often used for treatment. One of the main problems with these implants is the high revision rate, with physiological subluxation being one of the reasons. Therefore, OT Medizintechnik GmbH has developed a RHP that aims to restore the radiocapitellar column by a high congruency of the articulating surfaces. To evaluate whether the design reduces the tendency of subluxation and to compare the subluxation forces with multiple designs, biomechanical testing is needed. The aim of the study is to design and validate a test rig to investigate the subluxation resistance of an anatomically adapted (RC M), a patient-specific RHP (RC PSI) vs. the benchmark.

Methods: The testing was performed using an Uniaxial Test Machine (Instron). The test rig determines the resistance against subluxation in the anterior-posterior direction. An axial joint compression force (25N) between the humerus model and RHP longitudinal to the radial shaft axis was statically applied through a suspended weight plate. The RHP was aligned with the anatomical model of the capitulum in neutral position. Both the capitulum and the RHP were fabricated via rapid prototyping (Ender 3 Max Creality, PLA). The study was performed with a path-controlled quasi-static testing procedure. The head, mounted on a 0.5kN load cell, was translated from the centered position to the rim of the fovea (10 mm/min). This was repeated for 3 cycles. To evaluate how well the RHP are theoretically adapted to the anatomy, the fitting accuracy of each RHP to the surface of the anatomy was examined (GOM Inspect).

Figure 1 [Fig. 1]

Results and conclusion: Each RHP was tested three times. The maximum absolute error of 0.73N and relative error of 4.19% indicate that the study has a repetitious accuracy. A comparison of the determined peak subluxation forces and the surface analysis shows a strong correlation. The more the RHP resembles the anatomy, the higher the resistance against subluxation. Regarding the geometric properties of the RHP, a strong correlation between subluxation force and depth as well as curvature of the fovea becomes apparent. Under the given limitations, the RC PSI seems to have the highest resistance against subluxation (22.97N). For validation of the test procedure, the forces and force-position curves were compared to recent literature [1]. The comparison suggests that valid results can be determined with the test rig. For the further procedure, the number of test runs is increased, and different anatomies are used to confirm the obtained results.

Table 1 [Tab. 1]