Proximal/Distal Mismatch: Type A and C Femurs

Abstract

Femoral deformity in primary total hip arthroplasty can make restoration of joint biomechanics difficult. The S-ROM (DePuy Orthopaedics Inc., Warsaw, Ind) prosthesis’ modular design allows for independent matching of diaphyseal and metaphyseal anatomy. Corrective osteotomy may also be performed because the stem achieves both proximal and distal stability, which resists the micromotion and torsional loads that would otherwise prevent ingrowth. Seventy-three hips were reviewed at a minimum of 10 years’ follow-up. Osteolysis was common, occurring in 55% of hips, but no osteolysis distal to the stem/sleeve junction was present. At mean 12 years’ follow-up, the stem remains stable and no failures of the modular junction have been reported.

The human femur demonstrates a variety of anatomic relationships that fall within the scope of “normal” anatomy. Noble et al¹ recorded measurements from 200 cadaveric femora and found no constant relationship between the size and the shape of the proximal femoral metaphysis and those of the diaphysis. A mismatch between the size and shape of the proximal femur and the size and contour of the distal femur is to some degree within the norm of femoral anatomy. Total hip arthroplasty (THA) with proximally fixed femoral components must accommodate this variety to achieve reliable fixation and restoration of hip biomechanical relationships. In contrast, distally fixed stems achieve stability by close apposition of stem and bone in the metaphysis, essentially ignoring the geometry of the proximal femur. The proximal stem shape is less important to fixation and can be focused on restoring offsets. This method can be successful, both in the short- and long-term, but has the drawbacks of causing atrophy of the proximal bone from stress shielding, as well as creating a significant challenge in stem removal should revision become necessary.

Figure 1: AP radiographs of an S-ROM THA at 2 years (A) and 10 years (B) postoperative. Note the cortical streaming or spot welds, indicative of osseointegration according to Engh,7 at the inferior medial and lateral sleeve margins as well as at the lateral superior margin.

Stems that rely on fixation proximally must achieve excellent proximal host-bone contact, often referred to as “fit and fill.” Reliable bone ingrowth with a proximally coated stem requires not only excellent fit and fill of proximal host anatomy, but also resistance to micromotion and rotational torque. Optimally the stem must also re-establish anatomic hip biomechanics to best normalize gait, provide stability, and minimize wear. Unique design features of the S-ROM (DePuy Orthopaedics Inc., Warsaw, Ind) femoral component give a reconstructive surgeon the ability to achieve proximal fit while also restoring anatomic hip biomechanics and correcting deformity. Because the modular components of the S-ROM stem allow independent matching of the proximal and distal anatomy, the stem accommodates a variety of anatomical differences, both the wide “normal” variation as defined by Noble et al¹ and the reconstructive challenges of the severely altered anatomy encountered in situations such as developmental dysplasia of the hip or post-traumatic arthritis. There are >10,000 possible combinations of distal stem diameter, proximal stem body, proximal sleeve size, and neck length to accommodate the range of femoral shapes and offsets.

Bone ingrowth occurs only in the absence or near-absence of micromotion (Figure 1). Pilliar et al² reported on mechanical testing of porous-coated femoral and dental implants retrieved from laboratory animals and subjected to tension and compression loading (femoral implants) and histologic analysis (femoral and dental implants). To achieve bone ingrowth, they concluded that a maximum acceptable amount of micromotion was 28 microns, while micromotion of >150 microns was likely to produce, at best, fibrous ingrowth. With the current trend toward earlier return to activities of daily living with minimal weight bearing limitations in the early postoperative period, the clinical implication for stems that do not resist micromotion could be failure to achieve long-term stability and early subsidence.

A key element of resistance to micromotion is an implant’s torsional stability. Non-axial loads, such as arising from a chair or stair climbing, produce the most micromotion, because these movements put rotational stress on the implant.³ An implant’s ability to resist torsion directly affects the likelihood of bony ingrowth. Burke et al⁴ showed that despite stability in simulated single-leg stance, non-cemented implants experience movement of up to >200 microns with simulated stair climbing. A press-fit implant must, therefore, resist torsion to achieve bone ingrowth. Ohl et al⁵ performed laboratory testing on the S-ROM stem in three implantation configurations. The first group underwent the standard implantation technique, in which the femur was reamed proximally for a tight sleeve fit and under-reamed by 0.5 mm for a tight fit of the stem distally. The second group was prepared with the proximal femur again reamed for a tight fit of the sleeve but with the distal femur over-reamed by 2 mm for a loose distal fit. In the third group, the stem was implanted without the sleeve for a tight distal fit but no proximal contact. These three configurations were subjected to torsional loads. Mean load to failure was significantly greater for the stems that were tight both proximally and distally. Rotational micromotion was decreased by proximal tightness, but distal fixation was necessary for resistance to permanent rotational displacement. Most press-fit stems rely solely on the proximal femur for stability and do not have the added benefit of this distal resistance to rotational displacement.

Results At 10 Years’ Follow-up

A multicenter retrospective study was conducted to determine the clinical and radiographic results after primary THA with the S-ROM femoral component. The results of 175 hips at mean 5.1 years’ follow-up have been previously published.⁶ In that series, at medium-term follow-up, 98% of stems demonstrated stable bone ingrowth according to the criteria of Engh.⁷ Two stems demonstrated stable fibrous ingrowth, and one stem was loose. Osteolytic lesions were found in 12 (7%) hips. No evidence of osteolysis distal to the stem/sleeve junction was present. The clinical and radiographic outcomes were examined in relation to patient age, cortical index, filling of the distal part of the canal by the stem, and orientation of the femoral component. Cortical index describes the relationship of proximal femoral endosteal geometry to that of the diaphysis 10 cm below the lesser trochanter.⁸ Femora with a cortical index >0.55 (type A) are typically seen in young patients with a champagne-glass configuration, whereas osteoporotic femora have lower indices (<0.40, type C) and more of a stovepipe configuration. No differences based on these criteria were detected, suggesting that this implant works well regardless of age or degree of osteoporosis (Table). Also, some undersizing of the implant and orientation beyond neutral did not affect clinical or radiographic outcome. No failures of the modular junction were reported.

TABLE Association of Preoperative Cortical Index with Clinical and Radiographic Outcomes Cortical Index Number of Hips Postoperative Harris Hip Score Points Average ± Standard Deviation (%) Spot Welds Number of Hips (%) Corticocancellization Number of Hips <0.40 13 89.5 ± 6.5 4 10 0.40-0.55 90 90.0 ± 10.3 49 (54) 74 (82) >0.55 58 93.4 ± 5.6 29 (50) 45 (77) no data 14 n/a n/a n/a

This cohort was re-examined at minimum 10 years’ follow-up. Seventy-three hips met study criteria. At mean 11.9 years (range: 10-14 years), the S-ROM femoral component continued to function well. Harris hip scores remained mostly good to excellent with a mean of 86 points at latest follow-up (range: 37-100 points).⁹ Pain scores also remained stable with a mean at latest follow-up of 39.5 points (range: 10-44 points).

No new loose stems were found. The two hips that had been considered to have stable fibrous ingrowth at mean 5 years both continue to be stable at approximately 9 and 11 years’ follow-up.

Figure 2: AP pelvis of a Crowe type III dysplastic hip (A) and 12 years status post THA with femoral shortening osteotomy. The osteotomy allowed the placement of the acetabular component at an anatomic location without overlengthening the leg. The osteotomy is now completely healed and impressive spot welds indicate a well-ingrown stem (B).

The rate of osteolysis increased dramatically from 7% of hips at mean 5 years to 55% of hips at mean 10 years. Independent radiographic review found osteolytic lesions were limited to Gruen zones 1 and 7 and to the greater trochanter¹⁰; no osteolysis was reported distal to the stem/sleeve junction. A variety of acetabular components, many with gamma-irradiated-in-air polyethylene <5 mm thick coupled with 32-mm headballs, were used. These are known risk factors for wear-related osteolysis, and reports with other stems and similar types of polyethylene have also shown similarly high rates of osteolysis.^11-13 Significantly more osteolysis was found in hips with longer follow-up and those with thinner polyethylene. Osteolysis rates were also significantly different based on the acetabular component implanted. Osteolysis was found in only 11% of hips with an APR cup (Intermedics Orthopedics, Austin, Tex) versus 57% of the hips with a Joint Medical Products cup (Stamford, Conn). These differences support that the high rate of osteolysis seen in this series is not related to the modular junction but rather to the poor quality polyethylene implanted.

Twenty-two re-operations were performed on the original study cohort, mostly secondary to polyethylene wear and osteolysis. Seventeen of these revisions were either isolated acetabular revisions with exchange of the femoral headball at the time of re-operation or headball liner exchanges. In three hips, the femoral stem was also exchanged due to concern about femoral head-neck fretting. The well-ingrown modular sleeve was not disturbed, and a new stem was inserted. In the earlier series, one stem and one sleeve were revised for mechanical loosening. One total hip has been resected for late sepsis. In this longer follow-up series, no femoral prosthesis demonstrated subsequent mechanical loosening.

S-ROM in Proximal/Distal Femoral Mismatch

The S-ROM femoral component accommodates a variety of anatomic deformities without compromising its long-term success. Our center has used the S-ROM prosthesis in >1500 patients with a variety of diagnoses including 80 cases of developmental dysplasia of the hip with good results. An extreme example of femoral proximal/distal mismatch is encountered when reconstructing the dysplastic hip. Excessive proximal anteversion and significant leg length discrepancy are commonly encountered in a dysplastic hip.

Figure 3: A valgus osteotomy for femoral neck fracture nonunion has resulted in a significantly valgus proximal femur in this 47-year-old woman. Osteonecrosis is also present (A). The S-ROM sleeve has been placed to best contact host bone and, therefore, is rotated in relation to the stem approximately 180° from neutral (B).

The S-ROM allows control of femoral component anteversion because the sleeve’s rotational position and stem version are independent of each other. Because the S-ROM provides early rotational stability, a surgeon can also perform a femoral shortening osteotomy to approximate leg lengths while placing the acetabular component in a more anatomic location (Figure 2). The femur distal to the osteotomy is well controlled by the sharp stem flutes without resorting to a step-cut osteotomy, simplifying the surgical procedure.

The post-traumatic arthritic hip also often presents with significant bony deformity that is difficult to accommodate with a non-modular stem. For example, after a valgus osteotomy, the proximal femur’s anatomical relationship to that of the distal femur is significantly altered (Figure 3). The S-ROM allows a surgeon to both accommodate and correct this deformity.

The S-ROM femoral component’s unique design features provide a reconstructive surgeon with enhanced options for the correction of femoral deformity. Despite a high rate of periprosthetic femoral osteolysis, the S-ROM stem remains stable at mean follow-up of 12 years. The osteolysis rates are comparable to those of other reported studies at similar follow-up. The absence of osteolysis distal to the porous-coated sleeve suggests that an intact implant-bone interface between the femoral sleeve and the surrounding host bone has been attained. No failures of the modular junction have been reported. The S-ROM femoral component is an essential part of a reconstructive surgeon’s armamentarium. No adverse effects have been associated with the additional modular junction.

References

1. Noble PC, Alexander JW, Lindahl LJ, et al. The anatomic basis of femoral component design. Clin Orthop. 1988; 235:148-165.
2. Pilliar RM, Lee JM, Maniatopoulos C. Observations on the effect of movement on bone ingrowth into porous-surfaced implants. Clin Orthop. 1986; 208:108-113.
3. Crowninshield RD, Johnston RC, Andrews JG, Brand RA. A biomechanical investigation of the human hip. J Biomech. 1978; 11:75-85.
4. Burke DW, O’Connor DO, Zalenski EB, Jasty M, Harris WH. Micromotion of cemented and uncemented femoral components. J Bone Joint Surg Br. 1991; 73:33-37.
5. Ohl MD, Whiteside LA, McCarthy DS, White SE. Torsional fixation of a modular femoral hip component. Clin Orthop. 1993; 287:135-141.
6. Christie MJ, DeBoer DK, Trick LW, et al. Primary total hip arthroplasty with use of the modular S-ROM prosthesis. Four to seven-year clinical and radiographic results. J Bone Joint Surg Am. 1999; 81:1707-1716.
7. Engh CA, Massin P, Suthers KE. Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components. Clin Orthop. 1990; 257:107-128.
8. Dorr LD. Optimizing results of total joint arthroplasty. Instr Course Lect. 1985; 34:401-404.
9. Harris WH. Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am. 1969; 51:737-755.
10. Gruen TA, McNeice GM, Amstutz HC. “Modes of failure” of cemented stem-type femoral components: a radiographic analysis of loosening. Clin Orthop. 1979; 141:17-27.
11. Engh CA Jr, Claus AM, Hopper RH Jr, Engh CA. Long-term results using the anatomic medullary locking hip prosthesis. Clin Orthop. 2001; 393:137-146.
12. Hellman EJ, Capello WN, Feinberg JR. Omnifit cementless total hip arthroplasty. A 10-year average followup. Clin Orthop. 1999; 364:164-174.
13. Kim YH, Kim JS, Cho SH. Primary total hip arthroplasty with a cementless porous-coated anatomic total hip prosthesis: 10- to 12-year results of prospective and consecutive series. J Arthroplasty. 1999; 14:538-548.

Authors

Dr Christie and Ms Brinson are from the Southern Joint Replacement Institute, Nashville, Tenn.