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SECTION I: SYMPOSIUM: Papers Presented at the Annual Meetings of the Hip Society

Use of a Hydroxyapatite-coated Stem in Patients with Dorr Type C Femoral Bone

Kelly, Stephen, J*; Robbins, Claire, E; Bierbaum, Benjamin, E; Bono, James, V; Ward, Daniel, M

Section Editor(s): Hansseno, Arlen D MD, Guest Editor

Author Information
Clinical Orthopaedics and Related Research: December 2007 - Volume 465 - Issue - p 112-116
doi: 10.1097/BLO.0b013e318156bf96

Abstract

Stability of the femoral component is vital to achieve success in THA. Cementless femoral implants facilitate mechanical stability by providing a direct biologic interface between implant and bone. The use of ingrowth and on-growth surfaces promotes biologic and mechanical stability of the interface.

The long-term stability of cementless stems has been well documented in the literature.2-7,11,14,18 Studies with a minimum 10-year followup2,7 have demonstrated stable fixation at the interface. These results have minimized concerns of early mechanical failure and durability of implant fixation over time.

However, the use of cementless femoral components remains controversial in the subgroup of patients with wide patulous canals and poor bone quality. This subgroup was described by Dorr et al9 as Type C bone. This bone exhibits diminished cellular and structural qualities, which may compromise ingrowth/ongrowth of the femoral stem. Traditionally, femoral fixation in these patients has been obtained with polymethylmethacrylate bone cement.19 The use of cementless implants in this population presents a clinical challenge to obtain stability and a successful surgical outcome. While the use of cementless implants is increasing, few studies have demonstrated the efficacy of their use in this subgroup of patients.7,16 Previously published studies2,7 have demonstrated the long-term performance of this stem design. Therefore, we continue to use this design of stem for most of our patients.

We hypothesize high Harris Hip scores,13 high survival rates, high rates of femoral fixation, and low rates of subsidence and osteolysis can be obtained with a proximally hydroxyapatite-coated stem in Type C bone.

MATERIALS AND METHODS

We retrospectively reviewed a consecutive series of patients with Type C bone who underwent THA using a proximally hydroxyapatite-coated stem from September 1991 through November 1994. These patients were selected from the clinical database of the senior author (BEB). During this period the senior author performed 118 THAs using this stem design and 252 THAs using all stem designs. Of the 118 patients with this stem, 16 (14%) had Type C bone.

Nine of the 16 patients (10 hips) were men. The mean age of the patients at the time of surgery was 54 years. The average body mass index was 29. The principal diagnoses were osteoarthritis (15 hips), posttraumatic arthritis (one hip), and congenital hip dysplasia (two hips). We excluded one patient (two hips) who failed to present for clinical followup beyond 2 years post-operatively. At that followup visit, both of the patient's hips were functioning well. Multiple attempts to contact the patient failed. The remaining 15 patients (15 hips) were evaluated for this study.

One patient (one hip) underwent THA revision surgery of all components at 17 months secondary to sepsis and was excluded in the long-term evaluation of clinical and radiographic data. A second patient underwent revision of the acetabular components, both shell and liner as well as the femoral head at 12.8 years postoperatively. The patient's pain and function data were not available in the record before the revision. The patient's stem was well-fixed and retained. After excluding these two patients the remaining 13 patients had a minimum followup of 9 years (mean, 11.5 years; range, 9-14 years). A power analysis was not performed because of the retrospective nature of this analysis. The institutional review board approved this study.

The senior author performed all surgeries using a posterior approach. The femoral canal was prepared using the surgical technique as described for this stem.5 Briefly, the medullary canal is entered and then prepared with graduated taper reamers. Subsequently, the canal is broached using cutting broaches. Upon satisfactory completion of trailing, the final stem was seated in the medullary canal using an impactor. All patients received a proximally hydroxyapatite-coated femoral component of the same design. This stem, the Omnifit® HA (Stryker Orthopaedics, Mahwah, NJ) (Fig 1), has demonstrated excellent long-term survival in reported series.2,7 It is a straight titanium alloy stem with a proximal double-wedge design. On the anterior and posterior surfaces, it has machined normalization steps to improve stress transfer. The proximal third of the prosthesis is surface roughed using a plasma spray technique and is then coated with a 50-μm-thick coating of pure hydroxyapatite. The final surface roughness of the proximal ingrowth surface after coating is 4.3 to 8.1 μm. Distally, the stem is matte finished. All patients received a hydroxyapatite-coated acetabular shell with an increased peripheral radius. Patients received polyethylene acetabular liners and cobalt chrome femoral heads.

Fig 1
Fig 1:
An Omnifit® HA titanium alloy stem is shown.

Postoperatively, patients were allowed to bear weight as tolerated and were mobilized using our postoperative total hip protocol. The protocol allows for early postoperative mobilization with physical therapy, weight bearing as tolerated with an assistive device, and adherence to posterior total hip precautions. In the immediate postoperative phase, patients used crutches and progressed to use of a cane until no limp was present during walking.

Clinical outcome data were extracted from the patient's most recent postoperative visit. The Harris hip score13 was calculated from patient responses and surgeon documentation from the most recent postoperative visit.

Anteroposterior radiographs were obtained at the preoperative visit, immediately following surgery, and at each postoperative visit. Radiographs were obtained in a standardized fashion. Two independent reviewers (JVB, SJK) evaluated the radiographs. The first reviewer (JVB) identified patients as Type C bone by reviewing preoperative radiographs. A second reviewer (SJK) confirmed the definition of Type C bone by measuring the canal to calcar isthmus ratio (CC ratio) as described by Dorr et al.9 According to the Dorr classification, a CC ratio greater than 0.64 is considered Type C bone; the average CC ratio in the 15 patients was 0.74 (range, 0.65-0.95).

The second reviewer (SJK) also analyzed the long-term performance of the femoral stem using the most recent postoperative anteroposterior radiograph. The measured criterion for stem performance included subsidence greater than 0.5 mm and endosteal cortical erosion. Subsidence was measured using the technique described by D'Antonio et al.6 This measurement techniques compares the distance from the tip of the greater trochanter to the shoulder of the stem on the immediate post-op radiograph and the most recent radiograph using a scale rule. Other observations included stress shielding, osteolysis, cortical condensation, and cortical hypertrophy. These observations were performed using the clinical technique described by Engh et al.10 AP and lateral radiographs were analyzed for the presence of multiple radiographic features which have been demonstrated as indicative of a stable ingrown stem. These findings were used to generate a fixation/stability score as described by Engh et al,10 which classified stem stability as ingrown, suspected ingrown, fibrous ingrown, or noningrown. Features used to generate the fixation score include appearance of a stable bone implant interface and the presence of spot welds. The stability score was determined by evaluation for calcar remodeling, presence of a pedestal, migration, and the appearance of lucent lines. Cortical hypertrophy, stress shielding, and osteolysis were classified by radiographic zone as described by Gruen et al.12 The presence of heterotopic bone was classified using the Brooker classification.1

RESULTS

No femoral component was revised for aseptic loosening, migration, osteolysis, subsidence, or heterotopic bone formation. Four of the 13 patients (four hips) underwent revision of their acetabular liner and femoral head for osteolysis. This represented expected polyethylene wear of the acetabular liner. At the time of their revision surgery, all of the femoral components were well fixed and were retained. At the time of revision, each of these patients had been followed a minimum of 6 years (mean, 9.7 years; range, 6-11 years).

The median Harris hip score13 at 10-year followup was 94.5. Twelve of the 13 patients reported no pain or slight pain, and one patient reported moderate thigh pain at a mean 10-year followup. One patient reported moderate thigh pain and a Harris hip score13 of less than 70, which could not be related to physical examination or radiographic findings by the operating surgeon.

Of the four hips undergoing acetabular revision for wear, all demonstrated radiographic evidence of femoral stem bone ingrowth before revision, which was confirmed clinically at the time of revision. In the patient excluded for revision for sepsis at 17 months postoperatively, the radiographs demonstrated a radiographically stable stem, which was confirmed clinically at the time of explant of the components for infection.

Radiographic evaluation demonstrated no femoral fixation failures. All stems had bone ingrowth fixation. The majority of patients had evidence of spot welds visible in the hydroxyapatite coated portions of the stem. No patient had evidence of progressive lucent lines or evidence of subsidence.

At 10.7 years, one hip demonstrated femoral radiolucency in Gruen zones 3, 4, 5, and 7, as well as stress shielding and osteolysis in zones 1 and 7. This patient was asymptomatic. Stress shielding was defined as generalized loss of bone density within a Gruen zone. Osteolysis was defined as a circumscribed area of bone loss with clearly defined margins. Stress shielding was seen in four additional hips in zones 1 and 7. In all cases the stress shielding was mild. Three additional hips showed osteolysis in zones 1 and 7. Cortical hypertrophy of the femur was seen in five hips. This hypertrophy was observed most commonly in zones 3 and 5. A radiograph (Fig 2) represents a patient from this series who is 13.3 years from implantation of his L THA. The patient underwent THA of the contralateral side 2 years ago. The patient reports both hips continue to function well and are pain free. Note the evidence of bone ingrowth and spot welds evident on the radiograph demonstrating radiographic evidence of stable fixation.

Fig 2
Fig 2:
This radiograph of a 67-year-old male patient's left side is 13.2 years post-THA with an Omnifit® HA stem. The right side shows immediate postoperative THA.

DISCUSSION

The long-term performance of a proximal hydroxyapatite stem in Type C bone demonstrates excellent clinical and radiographic performance at a minimum of 9 years. In this series, there were no revisions of a femoral stem and all of the femoral stems demonstrated radiographic evidence of bone ingrowth and stability. Further, patient's clinical results as measured by the Harris hip score13 were also very satisfactory. These results are consistent with those reported with this stem design in other morphologic types of bone.

The primary limitation of this study is its small size and case series design. However, we believe it provides insight into the performance of this prosthesis design in a selected patient population with lesser quality bone. Certainly, it would be ideal to compare the 10-year results of this stem design in all patients who received it during the study period. In addition, it would have been preferable to analyze all patients with Type C bone who received a THA during this period. Technical limitations prevented us from being able to perform these analyses. During this study period we did not routinely record preoperative Harris hip scores;13 therefore we are unable to report patient's preoperative assessments. We identified no evidence of subsidence greater than 0.5 mm and believe the method described by D'Antonio et al6 sufficiently accurate for our purposes. We are familiar with reports that do not support reliability of subsidence measured at less than 2.5 mm. 8,15,17 recognize our radiographic measurements of subsidence may not be sufficiently reliable, but clinically and radiographically we see no evidence of stem subsidence.

The classification of Dorr et al9 attempts to classify host bone quality based on radiographic morphology. This classification is based on the assumption that specific radiographic measurements correspond to micrographic morphology. These morphologies are linked to overall host bone biology. Patients were separated into three classes based on a calculated CC ratio. Patients with a ratio of 0.57 were classified as Type A bone and had the strongest and most biologically active bone. This morphology has also been described as the “champagne flute.” Most patients in Dorr's series had a CC ratio of 0.59, or Type B bone, and had intermediate bone quality and activity. Patients with a CC ratio greater than 0.64 were classified as Type C or “stove pipe,” and are thought to have the least biologically active bone and the poorest bone for ingrowth. The average age of the patients in this study was 54 years, not the elderly group of patients who typically present with Type C bone. The demographic data of our patients varies from what was reported by Dorr et al9 in his initial study because our patient population is somewhat younger. This may represent our own bias at the time to use cemented stem in this patient population.

The use of cementless stems continues to grow in popularity. Capello et al2 reported a high survival rate, minimal long term morphologic femoral changes, few stem revisions, and radiographic evidence of ingrowth in a series with this stem at 15 years. However the long-term performance of this stem has never been demonstrated in Type C bone. The stem survival rate was 99.4% in their series of 166 hips at minimum 15 year followup. Our results at 10 years are comparable in a subgroup of patients who are believed to generally have poorer ingrowth potential.

A previously published study by Reitman et al16 attempted to document the performance of a taper design cementless stem in a subgroup of patients with Type C bone. In these patients, clinical results at 13 years were satisfactory but only a limited radiographic analysis was performed. Their patients demonstrated cortical hypertrophy and similar long-term issues with polyethylene wear. These results are also comparable to our own and are supported by the radiographic evidence of bone ingrowth.

At the time of the introduction of cementless stems, there was concern that satisfactory ingrowth could not be obtained in some patients. Therefore, many surgeons were hesitant to adopt use of these stems in patients where the host bone was believed biologically compromised. While the performance of cemented stems has been demonstrated by previous studies, the long-term performance of cemented stems has been compromised by stress shielding and osteolysis. In addition, intraoperative hypotension caused by polymethylmethacrylate is well described. We believe the use of cementless stems in patients with Type C bone offers a number of advantages for patients undergoing primary THA. Bone implant stress distribution in proximally fixed stems is more physiologic and mean operative time is reduced.

As with cemented stems, meticulous surgical technique is paramount for the success of these stem designs. Care must be taken when using noncemented stems in patients with Type C bone. Anecdotally, patients with Type C bone have weaker bone, thus proper preparation of the bone is important to support ingrowth and ongrowth. Care must be taken during broaching to avoid fracture. With meticulous surgical technique, no intraoperative fractures occurred in this series; however, it remains a risk at the time of broaching and implantation.

This study demonstrates long-term fixation of the femoral component can be attained in patients with Type C bone using a hydroxyapatite-coated cementless component. Although the biologic activity of Type C bone may be less than that seen in other classes of bone, the osteo-inductive properties of this stem with hydroxyapatite may provide a sufficient stimulus and milieu for bone ingrowth and ongrowth. This is supported by comparison to outcomes from previously reported series. Thus, a hydroxyapatite-coated femoral stem may be used successfully in primary THA for patients with Type C bone.

References

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