A cementless, elastic press-fit socket with and without screws: A 2-year randomized controlled radiostereometric analysis of 37 hips

Background

The acetabular component has remained the weakest link in hip arthroplasty regarding achievement of long-term survival. Primary fixation is a prerequisite for long-term performance. For this reason, we investigated the stability of a unique cementless titanium-coated elastic monoblock socket and the influence of supplementary screw fixation.

Patient and methods

During 2006–2008, we performed a randomized controlled trial on 37 patients (mean age 63 years (SD 7), 22 females) in whom we implanted a cementless press-fit socket. The socket was implanted with additional screw fixation (group A, n = 19) and without additional screw fixation (group B, n = 18). Using radiostereometric analysis with a 2-year follow-up, we determined the stability of the socket. Clinically relevant migration was defined as > 1 mm translation and > 2º rotation. Clinical scores were determined.

Results

The sockets without screw fixation showed a statistically significantly higher proximal translation compared to the socket with additional screw fixation. However, this higher migration was below the clinically relevant threshold. The numbers of migratory sockets were not significantly different between groups. After the 2-year follow-up, there were no clinically relevant differences between groups A and B regarding the clinical scores. 1 patient dropped out of the study. In the others, no sockets were revised.

Interpretation

We found that additional screw fixation is not necessary to achieve stability of the cementless press-fit elastic RM socket. We saw no postoperative benefit or clinical effect of additional screw fixation.During the last 5 years, several long-term reports on different methods to achieve primary stability in cementless sockets have shown excellent survival using aseptic loosening as the endpoint (Kim 2005, Kim et al. 2005, Firestone et al. 2007, Suckel et al. 2009, Pakvis et al. 2011).During the last 2 decades, we have used the cementless RM classic socket with good to excellent long-term results (Diks et al. 2005, Pakvis et al. 2011). Ihle et al. (2008) also reported good long-term results for this cementless, titanium particle-coated socket. This socket is based on the philosophy that an elastic polyethylene RM socket (approx 1,000 N/mm²), in contrast to a titanium-calcium phosphate rigid metal shell (approx 105,000 N/mm²), provides the elastic properties of acetabular bone (approx 500–6,000 N/mm²). The resulting physiological distribution of articular forces protects the acetabular bone and provides optimal conditions for ingrowth, with subsequent long-term component fixation. Due to stress shielding, rigid sockets may reduce the acetabular bone quality (Wright et al. 2001, Huo and Osier 2008). In comparison to femoral stress shielding, acetabular stress shielding results in osteolysis and component migration requiring revision surgery.The primary stability of the RM classic socket is achieved by 2 pegs and additional screw fixation; secondary stability is achieved from biological ingrowth into the titanium-particle coating. At our specialized orthopedic training hospital, we have encountered implantation difficulties, leading to a learning curve for optimal positioning of the pegs. In a primary series, this resulted in malpositioning of the socket and a high rate of early to short-term revisions (Diks et al. 2005). The new design of the peg-less, titanium particle-coated RM press-fit cup is expected to make it easier to implant (Figure 1).Open in a separate windowFigure 1.The cementless, titanium particle-coated RM press-fit socket.The effect of adding screws to a press-fit socket to optimize primary stability has been unclear. Some authors have seen no additional effect (Onsten et al. 1996, Thanner et al. 2000, Rohrl et al. 2006, Roth et al. 2006, Iorio et al. 2010), while other authors have described an extra stabilizing effect of the screws on the primary stability of press-fit sockets (Hadjari et al. 1994, Thanner et al. 1996). When additional screw fixation is used, some potentially negative effects on long-term survival must be accepted. The development of osteolytic lesions is believed to be the result of the transmission of articular pressure and of wear particles to the acetabular bone via the screw channels (Schmalzried et al. 1997).To our knowledge, only Thanner et al. (2000) have performed a radiostereometric analysis-based (RSA) randomized controlled trial (RCT); they found no effect of the additional screw fixation in a rigid metal-backed modular titanium-mesh HA- (hydroxyapatite-) coated socket.We performed an RCT to evaluate the stability of the cementless, RM press-fit socket with and without additional screw fixation. We used RSA to determine the stability in each group. We hypothesized that there would be a difference in stability between the cementless RM press-fit sockets with additional screw fixation and those without: that due to the elastic modulus of the RM press-fit socket without additional screw fixation, there would be a larger degree of migration shortly after surgery but that it would stabilize during the two-year follow-up.