THE EXCLUSIVE USE OF THREADED BICONE CUPS IN PRIMARY HIP REPLACEMENT SURGERY

Abstract

Intruduction: Cementless cup anchorage for total hip replacement is among the techniques widely accepted today. Attention focuses on hemispheric cups mimicking the anatomical shape of the bony acetabulum. However, the first-generation cementless hemispheric cups had a number of design flaws, which have meanwhile been attended to. We therefore never really turned away from threaded cups and began to implant a cone-shaped version of commercially pure titanium in January 1985. Introduced in January 1993, the redesigned version was unlike any other implant described in the literature in terms of its outer shape, the locking mechanism for the polyethylene liner, the cutting strength of its teeth and the thin wall of the cup shell. We wanted to know whether this implant was generally applicable for all primary THRs irrespective of the underlying anatomy, i.e. whether the exclusive consecutive use of this implant was justified.

Method: Between 1/1/1993 and 30/4/1994, 332 patients underwent primary surgery for osteoarthritis with threaded cups and titanium stems. These self-tapping double-cone cups made of pure titanium feature sharply cutting teeth for anchorage without screws. The PE liner locks into the titanium shell by a 4-level conical locking mechanism obviating the need for indentations for rotational adjustment of the liner. All cups and stems implanted during this period were uncemented. At 10 years plus, clinical and monitor-controlled radiologic follow-ups were conducted to evaluate changes in cup position, radiolucent lines, osseo-integration and revisions.

Results: 209 patients (63 males and 146 females; mean age at surgery: 62.6 years, range: 18.9 to 83.2 years) showed up for follow-up. 71 were dead, 36 without revisions were contacted by phone, 10 were lost to follow-up. The mean follow-up time was 10.2 years (range: 10.0 to 11.1 years). 2 patients had undergone cup revision, one for low-grade infection after 9.6 years and one for cup fracture after 5 years. With cup revision as the endpoint, the Kaplan-Meier survival rate was 99,2 % (CI: 96.6 to 99.8). Radiography showed altered cup inclination in two patients and radiolucent lines signalling absence of osseointegration in one patient. All other implants were stable clinically and radiologically. Gaps between the cup floor and the bone tended to be spontaneously obliterated by newly formed bone. Complete obliteration was observed even in cases with incomplete cranial implant coverage due to hip dysplasia.

Conclusion: The outcome of threaded double-cone cups at 10 years and more compares well with the best results achieved with other implants, particularly hemispheric cups. This documents that their unique design features have so far stood the test of time. It also shows that these cups have a place in all patients candidates for primary total hip arthroplasty.