Ceramic acetabular cups for total hip replacement part 7: How are range ofmotion and impingement influenced by the position of the centre of rotation and the CCD angle of the stem?

The range of motion (ROM) of total hip prostheses is influenced by a number of parameters. An insufficient ROM may cause impingement, which may result in subluxation, dislocation or material failure of the prostheses. In a three-dimensional CAD simulation, the position of the centre of rotati on and the CCD angle of the stem were investigated. Displacement of the centre of rotation of the femoral head may be due to we ar (PE cups) or to the design of the prosthesis (ceramic cups). Stems of wi dely differing design have been developed and implanted. The results of the present study demonstrate that the ROM is clearly reduce d by increasing penetration of the femoral head. At an inclination angle of 45 degrees, a depth of penetration of 2 mm restricts flexion by about 15', and a depth of penetration of 3 mm by about 30 degrees. At smaller angles of inclination the ROM is reduced and flexion and abduction are associated with an increased risk of impingement. With steeper acetabular cup inclinat ions, the risk of impingement decreases, but dislocation, the risk of rim f ractures (ceramic cups), and wear and penetration rates (PE cups) increase. The CCD angle of the stent should be oriented to the anatomical situation. At high CCD angles (>135 degrees), flexion is clearly limited, in particula r when there is penetration of the femoral head. For modern total hip arthr oplasty, prosthetic systems characterised by precise positioning of compone nts, minimum wear, slightly recessed inserts, and appropriate CCD angles sh ould be used.