A clinical study of uncemented hip arthroplasty : radiological findings of host-bone reaction to the stem

Abstract

In total hip arthroplasty (THA), the main reason for failure leading to revision is aseptic loosening. Both mechanical and biological effects act in the loosening process, the factors may be implant design, fixation mode or technique, as well as biological factors unique to the individual. Younger and more active patients have a worse outcome compared to older patients. In an attempt to reduce the incidence of aseptic loosening, uncemented implants were introduced.

The load that is transferred over the artificial joint is taken up by the femoral implant and a significant fraction of the load is transferred further distally to the host bone. An adaptive bone remodelling occurs and is commonly discussed in terms of stress-shielding. The general aim of the study was to evaluate the outcome after operation with an uncemented femoral stem and how the femoral host bone reacts in terms of adaptive remodelling.

Material and Methods: Two types of tapered uncemented stems were evaluated. One stem with the surface coated with polytetrafluoroethylene, designed to enhance fibrous tissue ingrowth (Anaform®). One titanium stem with a proximal porous and hydroxyapatite coating, designed for bone ingrowth (Bi-Metric®). Clinical outcome was evaluated with the Harris Hip Score. Changes in bone mineral density (BMD) were assessed by Dual-energy X-ray Absorptiometry (DEXA). Bone metabolism was assessed by scintigraphy and radiological changes were noted using the criteria of Engh et al.

Study I: The two types of stems were compared eight years postoperatively. The stem coated with polytetrafluoroethylene showed signs of instability and a more generalized distribution of bone resorption. The proximally coated stem showed most pronounced resorption proximally, despite only proximal coating.

Study II: In a randomized controlled study, immediate and late weight-bearing after uncemented THA, were evaluated. Immediate weight-bearing had a positive effect on BMD around the prosthesis. No adverse clinical or radiological sign were detected.

Study III: In 138 patients with unilateral uncemented stems, BMD was evaluated after a mean of 41 months. The size of uncemented stems was found to correlate with periprosthetic BMD. Bone loss in the proximal periprosthetic regions was significantly associated with larger stem sizes.

Study IV: 104 hips in 95 patients were followed for a minimum of 10 years after uncemented THA with the Bi-Metric® stem. Excellent radiological results were noted for the stem. No stem was revised and no sign of loosening was detected, despite an inferior result on the acetabular side with 23 failures. However, progressive remodelling was noted on the femoral side, also between five and ten years.

Study V: Two groups of 14 patients were followed for 10 and 14 years with DEXA and radiographs after implantation of the Bi-Metric® stem. At follow-up, a continuing decrease in BMD up to 14 years was noted in the calcar region.

In conclusion, fibrous fixation of an uncemented stem is insufficient. A tapered titanium stem, porous and hydroxyapatite coated in the proximal region, showed signs of bone ingrowth and excellent clinical result after 10-14 years. Radiological signs of stress-shielding and proximal periprosthetic bone resorption are continuous up to 14 years.