The S-ROM Stem and Its Use With Alternative Bearings

Abstract

Many long-term studies of total hip arthroplasty (THA) show excellent results. This long-term success depends on many factors, including implant fixation and bearing surface wear. As THA is commonly performed on patients with a steadily increasing life span and activity level, the issue of wear has become critical. Advances in the wear properties of polyethylene have been significant, but, in the search for low long-term wear rates, hard bearing surfaces are frequently used.

Both metal-on-metal and ceramic-on-ceramic bearing surfaces have been used for >30 years. High early failure rates of metal and ceramic prostheses, coupled with the success of metal-on-polyethylene articulations, led to a diminishing enthusiasm for the use of prostheses. After long-term follow-up and continued research into the wear rates and properties of hard bearings, there has been a renewed interest in the use of metal and ceramic prostheses.

Figure 1: Fractured ceramic femoral head and liner.

A close look at the initial metal and ceramic bearing prostheses reveals reasons for failure.^1-8 Poor fixation of the acetabular cup and femoral stem, and problems shared by many components of this generation plagued early prostheses. In addition, manufacturing tolerances were not as well understood or controlled during development. Imprecise mating of the head and liner, as well as poor head clearance, led to seizing or fracture of components (Figure 1). The manufacturing of ceramic was not well refined (as demonstrated by large grain size and inclusions), leading to higher fracture rates. Both the Mittlemeier and McKee-Ferrar prostheses had poor head-neck ratios and lacked significant femoral offset. These design issues led to a high risk of component impingement with patient activity. In metal-on-metal implants, impingement led to a higher rate of implant loosening. Patients with ceramic-bearing prostheses also had a high rate of loosening with impingement and the catastrophic consequence of ceramic chipping and fracture (Figure 2). An understanding of these prostheses’ modes of failure, combined with excellent long-term wear rates in the surviving implants, led to resurgence in their development and use.

Hard bearing surfaces offer many potential advantages over polyethylene, including low wear rates. Conventional polyethylene has wear rates of 200 microns per year. Cross-linked polyethylene has been used since 1999, and in vitro studies have shown significant reductions in wear rates over standard polyethylene and early clinical results have been favorable.⁹ Cross-linked polyethylene has lower wear rates, in the range of 20 microns per year, than conventional polyethylene. Metal-on-metal articulations have shown wear rates of 2-4 microns,⁵ and ceramic-on-ceramic bearings have shown the lowest wear rates of <1 micron per year.¹⁰ The benefit of these lower wear rates is the longevity of the bearing surface and the lack of particulate debris leading to osteolysis, which has rarely been prevalent in long-term follow-up of hard-bearing surfaces that have survived implant design shortcomings. The other advantage of hard bearing surfaces is the ability to allow large femoral head options. Large femoral head diameters, combined with optimal clearance and surface finish, allow for better lubrication of the joint and, therefore, lower wear rates than smaller heads. The larger femoral heads allow for a greater range of motion prior to prosthetic impingement,¹¹ which should lead to an overall lower rate of dislocation.

Figure 2: Chipped ceramic liner.

When compared to polyehtylene liners, the lack of modular liner options with ceramic and metal inserts is a disadvantage. Most acetabular systems offer neutral, 10°-lipped, 20°-lipped, or face-changing polyethylene liners, as well as lateralized options. These options are used to minimize prosthetic and soft tissue impingement, therefore, maximizing range of motion. They can also be used to subtly adjust leg length. All of these options are lost with hard-bearing liners. This loss of flexibility on the acetabular side makes the mating of the S-ROM stem (DePuy Orthopaedics Inc., Warsaw, Ind) with a hard-bearing surface, a perfect match.

The S-ROM stem is a two-piece modular stem with proximal and distal fixation, and proximal ingrowth. The stem and sleeve are placed independently of each other, allowing for optimal version of the femoral stem regardless of the proximal femoral anatomy. Many young patients who require a hard-bearing surface have proximal deformities that do not allow ideal femoral anteversion with a one-piece, broached femoral stem, including the diagnoses of slipped capital femoral epiphysis, dysplasia, Perthes disease, and post-traumatic osteoarthritis.

Optimizing femoral anteversion leads to greater range of motion that is an advantage in this more active group of patients, but, more importantly, it decreases the risk of prosthetic impingement. In a hard-bearing surface, impingement can lead to either ceramic chipping or an unintended mode of wear in a metal, or metal-lipped liner.¹² This modular stem with independent control of version offers a distinct advantage in this situation.

In addition to its advantages in controlling version, the S-ROM also offers many different neck options; up to six neck options exist for each distal stem diameter. These options allow surgeons flexibility to match patients’ offset and leg length precisely. Offset and leg length can be controlled independent of each other. This serves to decrease the risk of impingement and, therefore, increases range of motion. The likelihood of surgeons needing a skirted femoral head decreases significantly with these neck options. A skirted femoral head in a metal-on-metal bearing surface is also a less than ideal situation for all of the reasons of prosthetic impingement. In a ceramic implant, fewer head options tend to exist due to the inability to make skirted heads. Adjusting the length on the femoral component side is a significant advantage. Combining the neck, sleeve, and stem options give surgeons >10,000 options to create the ideal situation for matching patients’ offset and leg length, minimizing impingement, and maximizing range of motion.

Figure 3: S-ROM stem with altered stem sleeve anteversion.

While using the S-ROM stem, the ideal position for sleeve placement in the calcar is different than the ideal positioning for stem anteversion (Figure 3). When using a broached one-piece stem, surgeons can either attempt to cut the calcar with the broach to subtly improve this position, or accept the less than ideal anteversion and compensate for this on the acetabular side with either cup position, or by using a lipped or face-changing liner. When reviewing a consecutive series of 100 S-ROM total hip arthroplasties, the author found a wide range of sleeve positions. Measuring each sleeve position upon placement, only 47% of patients had 15° of anteversion. Another 27% of patients had sleeve position between 5° and 15° of anteversion. Eighteen percent of patients had neutral anteversion, 1% was retroverted, and 6% of patients had >20° of anteversion. Using only neutral liners and mostly 28-mm femoral heads in this series of patients, each was checked for intraoperative range of motion and impingement. Only one early postoperative dislocation existed that resolved with closed reduction and had no subsequent dislocations, leaving a 1% dislocation rate. The ability to optimize the range of motion using only neutral liners is an advantage of the S-ROM stem with hard bearings.

In the development of the prototype high-demand hip, ideally the prototype would have excellent long-term fixation, negligible long-term wear, and the range of motion would minimize any risk of impingement and dislocation in active patients. The mating of the S-ROM stem and hard-bearing surfaces provides these advantages.

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Author

Dr Politi is from the Cardinal Orthopaedic Institute, Columbus, Ohio.