Modular Junctions

Abstract

A concern about modular stems is that the taper will not provide an adequate seal, causing the stem distal to the taper to become part of the effective joint space. In this study, 380 primary patients and 309 revision patients were treated with a proximally modular stem. Follow-up was between 5 and 17 years (mean 9.4 years). Three patients only had distal osteolysis. All three patients had failure of proximal bone ingrowth. It was concluded that a Morse taper provides an adequate seal or gasket.

Modular implies that one part can be locked to another interchangeable part. The concept of modular heads was developed in the 1970s to allow aluminum oxide ceramic heads of different lengths to be locked to a chrome cobalt stem. The ease and convenience of modular heads was so striking that today most hip systems use modular heads. Although the size of the taper varies from 10/12 to 14/16 and most tapers are 6°, sufficient differences in geometry exist. Therefore, a modular head from one company should not be used with a stem from another manufacturer.

Modular stems have been developed during the past two decades. Most modular stems use a Morse taper lock mechanism similar to the mechanism used by modular heads. The locking may or may not be positive (positive locking mechanism occurs when the stem is loaded and the male part of the taper is forced more tightly into the female part, enhancing the locking mechanism). Greater hoop stresses are generated in the female part, but as these stresses are frozen, the female part does not experience cyclic hoop stress loading.

Distally modular stems have underperformed and have been withdrawn from the market. Problems currently exist in mid-stem modular stems because the mid-stem is the part of the hip stem loaded most heavily in bending. Some proximally modular stems are available. The S-ROM (DePuy Orthopaedics Inc., Warsaw, Ind) is a proximally modular stem with a follow-up of almost two decades.¹

The stem is made of 6A14VaTi, is circular, and is canal filling distally. Distal thin, sharp 0.6-mm flutes are designed to engage the endosteal cortex and provide rotational stability. A full distal canal fill >5 cm provides angular stability. The proximal sleeve comes in varying geometries, has a single bead porous coating layer, and is stepped to convert shear loads to compression loads as much as possible. The sleeve provides resistance to sinkage.

The author is not aware of any patient in whom stem/sleeve dissociation occurred when the taper was locked correctly.

As long as the liner is polyethylene, wear debris particles, which induce osteolysis, affect the acetabulum and the proximal femur because they are part of the effective joint space. A question of interest is: “Does the taper junction provide an adequate seal to prevent the occurrence of distal osteolysis?” In an attempt to answer this question, the author has re-examined patients whom he treated with S-ROM stems.

Materials and Methods

All patients treated with S-ROM by the author have been followed up on a regular basis, annually when possible and through letters when patients failed to return for follow-up for >3 years. All patients have been examined clinically and radiologically, and the results were filed. For the purpose of this report, all files documenting patients with >5-years’ follow-up have been examined. A 5-year follow-up was chosen because osteolysis is seldom seen in patients with <5-years’ follow-up.

In this study, 380 primary patients and 309 revision patients were observed. The mean age for the primary patients was 54, and the mean age for the revision patients was 71. The follow-up was 5-17 years, with a mean of 9.4 years.


Figure 1. A young woman with routine osteoarthritis of the hip (A). A straightforward total hip replacement was performed in 1989 (B). At 1 year, a radiolucent line around the sleeve and some subsidence indicates a failure of ingrowth (C). By 1992, osteolysis distal to the sleeve is obvious (D). Nine years post-revision, no evidence of osteolysis is present (E).

Results

Three patients had distal osteolysis, including two primary patients and one revision patient. One of the primary patients died of causes unrelated to her hip surgery before she became symptomatic. The other primary patient had an area of expanded osteolysis resulting in a periprosthetic fracture through the area of osteolysis. She was revised to a long stem. The area of osteolysis healed, and 9 years later she remains asymptomatic (Figure 1). In the revision patient, the acetabular component, which was the original and had adequate fixation at the time of revision, developed severe osteolysis and loosened at 7 years. At the revision, the stem was tight. By that time, the lucency around the sleeve had disappeared, probably as a result of the stem subsiding, but the subsidence was immeasurable. A window was opened in the area of osteolysis, and a bone graft was inserted. The patient has since healed.

Discussion

In all three patients with distal osteolysis, initial failure of ingrowth into the sleeve resulted in a radiolucent line between the sleeve and bone. This line was presumably filled with fibrous tissue. However, in these three patients, the fibrous tissue did not prevent the distal end of the stem from becoming part of the effective joint space, allowing access of polyethylene debris that resulted in osteolysis.

In the first year after the introduction of the S-ROM stem, a vertical groove was present in the stem, providing an opening in the taper junction. Sixteen of these stems were inserted using a threaded acetabular component, and seven cups were loosened and revised. Four patients developed distal osteolysis. In three of these patients, the original sleeve was left because it was solidly ingrown. The distal canal was curetted, and a new stem without vertical grooves was inserted. The distal osteolysis healed without grafting. In one patient, the stem was revised to a long stem. Five of the remaining patients are now at 18 years and show no evidence of distal osteolysis.

Given the low incidence of distal osteolysis, it appears that the stem/sleeve junction is an adequate gasket or seal for excluding the lower part of the stem from the effective joint space, at least in the 17-year timeframe studied.

Assuming that the stem/sleeve junction is an adequate seal, the lack of distal osteolysis in primary patients is understandable because a fit between sleeve and metaphyseal bone is easy to achieve and results in almost universal bone ingrowth. However, the lack of distal osteolysis in revision patients is more difficult to explain because obtaining a complete seal between the sleeve and the metaphyseal bone is frequently impossible, at least at the upper end of the sleeve. Perhaps the distal end of the sleeve obtains adequate fill, or the cancellous bone in the proximal femur regenerates quickly enough to exclude the distal end of the stem from the effective joint space.

The stem/sleeve junction in the S-ROM proximally modular stem provides an adequate seal or gasket for at least the first two decades of service life, and distal osteolysis is rare.

Reference

Cameron HU. The 3-6 year results of a modular noncemented low-bending stiffness hip implant: a preliminary study. J Arthroplasty. 1993;8:239-243.

Author

Dr Cameron is from The Orthopedics and Arthritic Institute of Sunnybrook and Women’s College Health Sciences Centre and The University of Toronto, Toronto, Canada.