[Butel's isoelastic hip prosthesis. A limited prospective study]

A limited prospective study was undertaken at the Centre Hospitalier de l'Université Laval to assess the outcome of 21 isoelastic prostheses described by Butel.

The femoral component consisted of a one-piece forged stainless steel stem with four metallic rods ranging in diameter from 11 to 18 mm, in 1 mm increments. The two lateral and medial rods, 20 cm in length, were linked distally. The 2 mm gap between the medial and lateral rods was obliterated at insertion, allowing the component to be firmly seated in the femoral canal. Twenty-one Butel isoelastic prostheses were inserted in 20 patients (14 men and 6 women with an average age of 51 years). The preoperative diagnosis was aseptic total hip prosthetic loosening in three cases, rheumatoid arthritis in five cases, avascular necrosis in six cases and degenerative arthritis in six cases. A standard postero-lateral approach was used in all cases. Postoperatively, the patients were mobilized immediately, with partial weight bearing on the affected side for six weeks.

Follow-up ranged from 24 to 34 months with an average of 28 months. One patient, who developed a late infection, was excluded from the study. Nineteen patients with 20 prostheses were reviewed clinically and roentgenographically by independent observers in the absence of the senior author. Clinically, the patients rated 11 to 58 on the preoperative Harris scale with an average of 34 while they rated 39 to 96 postoperatively with an average of 67. Eight patients were below 70 on the Harris scale postoperatively and were rated unsatisfactory. Eleven patients complained of postoperative thigh pain which was still present in three patients after two years. None complained of pain related to the acetabular component. On roentgenograms, prosthetic seating seemed to be inadequate in 14 cases but with good clinical results in 9 cases. Seating seemed to be adequate in six cases but with an unsatisfactory outcome in three cases.

The concept of isoelasticity is that the implant and bone should deform as one unit. However, clinical results of an implant must be validated by investigators who are not developers of their own implant. The results in this series do not corroborate with those reported by Butel. A number of technical problems occurring at or after surgery precluded the safety, efficacy and durability of the surgical procedure and its successful outcome. A substantial incidence of thigh pain was found in this series. This has also been noted by a variety of authors using femoral designs with a large intramedullary rod which has less torsional stability than a flat wedge shaped femoral implant.

The isoelastic prosthesis used in this series did not fulfill the expectations and was considered marginal in regards to quality and efficiency.