Abstract
Since 1977, all total hip arthroplasty (THA) patients at Anderson Clinic have been considered candidates for porous coated stems, unless the geometry of the femoral diaphysis precludes initial press-fit stability. Over the last 23 years, more than 99% of all hip replacements at the Clinic have been cementless.
Among all primary THAs performed at the Anderson Clinic between 1977 and 1998, we are prospectively evaluating the outcome of 3363 porous coated stems. This includes 1800 extensively AML, 1098 extensively coated Prodigy and 465 proximally coated AML stems. Each of these femoral components features a beaded surface for bone ingrowth and a straight, cylindrical, non-tapered distal stem geometry.
Among the 2898 extensively coated stems, 17 (0.6%) have been revised including nine (0.3%) for failure to achieve initial bone ingrowth with subsequent clinical loosening. Among 465 proximally coated stems, 10 (2.1%) have been revised, including six (1.3%) for failed initial ingrowth and subsequent clinical loosening. Using revision for any reason as an endpoint, the probability of survival at 15 years was found to be 97.1 ± 1.2% for extensively coated stems and 96.5 ± 1.2% for proximally coated stems.
Due to a higher occurrence of thigh pain and a slightly reduced incidence of bone ingrowth with proximally coated stems, we currently prefer to use extensively porous coated stems for all patients. The advantage of extensive porous coating is that biologic fixation via osseointegration can occur over the entire length of the stem. Coating the distal part of the stem is particularly important because it is the distal part of the stem that fits and most consistently conforms to the shape of the femur. We have learned from our analysis of autopsy cases that the cortical bone of the femoral diaphysis demonstrates superior ingrowth characteristics and is of greater strength than cancellous bone.
Our analysis of serial radiographs has taught us that postoperative canal fill is the best predictor of subsequent bone ingrowth among extensively coated stems. Consequently, preoperative templating using standardised anteroposterior radiographs obtained with the femur in 20° of internal rotation and Lowenstein lateral views are used to estimate the size of the prosthesis that will fill the femoral canal. Extensively coated stems should obtain at least five and ideally 10 cm of diaphyseal “scratch-fit” fixation. For this reason, it is important to remember that the final selection of implant size is based not on the templating, but on the feel derived from preparing the femoral diaphysis with progressively larger diameter intramedullary drills that engage progressively longer segments of the intramedullary canal.
Among implants achieving bone ingrowth, we have not observed late aseptic loosening. We conclude that porous coated femoral stems offer a means to reliably obtain durable long-term fixation using a relatively easy, reproducible surgical technique.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
