Integral porous femoral stem

The Journal of Arthroplasty Vol. 12 No. 3 1997

Integral Porous Femoral Stem 5- to 8-Year Follow-up Study David R. Mauerhan, MD,* Joseph Mesa, MD,~- Alexia M. Gregory, BS,* and Jeffrey G. Mokris, MD*

Abstract: The outcome of total hip arthroplasty without cement was prospectively followed in patients who received a titanium taper wedge femoral component with a circumferential titanium plasma spray coating on the proximal surface (Integral femoral stem, Biomet, Warsaw, IN). One hundred ninety femoral components were implanted in 179 patients. Nine patients (9 hips) died prior to their 5-year examination, leaving 170 patients available for follow-up evaluation. Thirty-one of the 170 patients (33 hips) were lost at an intermediate follow-up period of 5 years (mean, 5.8 years, range; 5-8 years), giving a follow-up rate of 82% for patients and hips. The preoperative diagnoses included osteoarthritis (91 hips), avascular necrosis (38), post-traumatic arthritis (10), rheumatoid arthritis (4), congenital dislocation of the hip (3), and fractures (2). The 78 men and 61 w o m e n had a m e a n age of 55 years (range, 18-81 years) and a mean weight of 81 kg (range, 50-136 kg). Preoperative Harris pain and function scores for the hip were 19.9 (range, 0-40) and 55.6 (range, 12-84), respectively. At the most recent follow-up visit, the mean pain score increased to 41.2 (range, i 0 - 4 4 ) and the mean function score increased to 93.5 (range, 6-100). Thigh pain was present in 4% of the hips at the most recent follow-up visit. Radiographically, 99% of the femoral components demonstrated spot welds in the porous-coated zone. Two hips had demarcation of the femoral component in the porous-coated zone consistent with fibrous fixation. None of the femoral stems had been revised and there were no signs of aseptic loosening. Additionally, there were no cases of pain that could be directly attributed to the stem and there was no evidence of distal femoral osteolysis. It is concluded that the Integral femoral stem provides excellent clinical and radiographic results at intermediate follow-up periods. K e y w o r d s - arthroplasty, Integral femoral stem, uncemented hip prosthesis, arthritis.

C e m e n t l e s s f i x a t i o n of t h e f e m o r a l c o m p o n e n t in t o t a l h i p a r t h r o p l a s t y (THA) c o n t i n u e s to b e w i d e l y u s e d as a n a l t e r n a t i v e to c e m e n t e d f e m o r a l fixation. C o n t r o v e r s y r e g a r d i n g t h e o p t i m u m m e t h o d of f e m o r a l f i x a t i o n is d u e in p a r t to t h e d i s p a r i t y in l e n g t h of f o l l o w - u p p e r i o d in s t u d i e s e m p l o y i n g

Prom the *Miller Orthopaedic Clinic, Charlotte, North Carolina, and ~The Rothman Institute, Philadelphia, Pennsylvania.

Reprint requests: Gary M. Kiebzak, PhD, Director of Research Services, Miller Orthopaedic Clinic, 1001 Blythe Boulevard, Suite 200, Charlotte, NC 28203.

cemented versus cementless fixation. Although several studies have shown excellent long-term results with a cemented Charnley femoral compon e n t at 15 to 20 y e a r s [1,21, o t h e r c e m e n t e d f e m o r a l d e s i g n s h a v e h i g h e r r a t e s of a s e p t i c loose n i n g at s h o r t e r f o l l o w - u p i n t e r v a l s [ 3 - 5 ] . Clinical trials of b i o l o g i c a l l y f i x e d c e m e n t l e s s c o m p o n e n t s of m u c h s h o r t e r f o l l o w - u p p e r i o d s s h o w v a r i a b l e a n d m i x e d results. C a l l a g h a n et al. reported 2-year results with the porous-coated c o b a l t - c h r o m e f e m o r a l i m p l a n t [6]. Clinically, t h e r e w a s a 1 6 % i n c i d e n c e of t h i g h p a i n a n d 2 8 % of

Integral Porous Femoral Stem

patients had moderate or severe limp; however, the average Harris clinical hip rating was 92. Heekin et al., in a 5- to 7-year review of the same prosthesis, reported Harris hip scores of 92 with a 15% incidence of continual thigh pain [7]. Radiographically, fibrous fixation was present in 1%, unstable fixation was present in 5%, and 5% of the femoral c o m p o n e n t s had subsided. M a l o n e y and Harris, in a matched-paired series of hybrid THA, reported a 20% incidence of radiographic loosening, 24% incidence of pain, and 16% incidence of femoral revision in the u n c e m e n t e d femur at 2 to 4 years [8]. Engh et al. have s h o w n good clinical and radiographic results with the AML c o m p o n e n t (DePuy, Warsaw, IN) at 10 to 12 years [9]. They reported 11-year survivorship of 91.8% in the original 166 single-sized AML stems. They also noted osteolytic reactions in 25 (34%) cases, which were confined to the calcar and greater trochanteric area with no distal lysis. The purpose of this study was to present our intermediate results from a series of patients w h o were followed for a m i n i m u m of 5 years after undergoing a THA with a titanium femoral compon e n t using cementless fixation.

Materials and Methods B e t w e e n July 1986 and December 1989, 190 prim a r y cementless THAs were performed in 179 patients using the Integral femoral stem (Biomet, Inc., Warsaw, IN), which was a tapered, collared, titanium c o m p o n e n t with a circumferential titan i u m plasma spray coating on the proximal 30 to 40% of the stem (Fig. 1). The acetabular components consisted of a threaded, titanium plasmasprayed hemispherical cup (32% of cases), a hemispherical titanium plasma-sprayed cup with screws (34%), or a bipolar c o m p o n e n t (34%). Femoral head sizes were 28 and 32 mm, the majority being titanium alloy. At follow-up evaluation there were 78 m e n (56%) and 61 w o m e n (44%) with a m e a n age of 55 (range, 18-81) and a m e a n weight of 81 kg (range, 50-136 kg). The surgery was p e r f o r m e d on 66 (45%) right hips and 82 (55%) left hips. The preoperative diagnoses were primary osteoarthritis in 91 hips, avascular necrosis in 38 hips, post-traumatic arthritis in 10 hips, r h e u m a t o i d arthritis in 4 hips, congenital dislocation of the hip in 3 hips, and fractures in 2 hips (Table 1). All surgeries were performed using a posterolateral approach. Sequential conical reaming and broaching were done with broaches undersized by

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Fig. 1. Integral femoral c o m p o n e n t profile; anteroposterior and lateral views.

0.5 m m to the real c o m p o n e n t to insure m a x i m u m fit and fill of the femoral canal. All surgeries were performed by two surgeons (D.R.M. and J.G.M.). The postoperative regimen included partial weight bearing for 6 weeks, followed by advancement to full weight bearing as tolerated by the patient. Of the 179 original patients, 9 (5%) died prior to their 5-year examination, and 31 (17%) patients (33 hips, 17%) were lost to follow-up evaluation, leaving 148 hips in 139 patients with a m i n i m u m 5-year follow-up period (mean, 5.8 years; range, 5-8 years). Multiple attempts were made to contact all patients lost to follow-up evaluation via certified mail and telephone calls at h o m e and at work. Seven patients refused follow-up evaluation because of inconvenience and stated that they were doing well. Three patients had revision of their failed threaded acetabular c o m p o n e n t s at

Table 1. D e m o g r a p h i c s of Patients With I n t e g r a l S t e m s Men/Women (n) Age (y) Weight (kg) Diagnosis Osteoarthritis Avascular necrosis Post-traumatic arthritis Rheumatoid arthritis Congenital dislocation of hip Fracture Follow-up period (y)

78/61 55 (range, 18-81) 81 (range, 50-136) 91 38 10 4 3 2 5.8 (range, 5-8)

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other institutions. Two were in nursing h o m e s and one was in prison.

Clinical Evaluation Pain and function were assessed by a certified physical therapist using a Harris hip rating before and 1, 2, 5, and 7 years after surgery. Scores of 90 to 100 were considered to be excellent; 80 to 89, good; 70 to 79 fair; and under 70, poor results. The patients were questioned about the presence or absence of groin and thigh pain at each clinical visit.

Radiographic Analysis I n d e p e n d e n t radiographic analysis was provided by an unbiased reviewer (J.M.) at a different institution w i t h o u t knowledge of patients' clinical scores. Each patient had an anteroposterior radiograph of the pelvis and hip and frog-leg lateral radiographs of the hip performed before surgery and, t h e n after surgery at 6 weeks, 1, 2, 5, and 7 years. Magnification correction factors were calculated for each film based on the ratio of the measured diameter of the prosthetic head to the actual k n o w n diameter. The immediate postoperative and most recent follow-up radiographs were reviewed for signs of femoral and acetabular c o m p o n e n t failure. Because the focus of the study was the femoral stem components, the radiographic analysis of the acetabular c o m p o n e n t s is not included in this report. Preoperative radiographs were evaluated for Dorr bone subtype [10]. Postoperative films were evaluated for ectopic bone according to the Brooker et al. classification [11]. Femoral component position was assessed using a fixed point of reference on the prosthesis and the femur (using the lesser trochanter). Subsidence was present if the c o m p o n e n t settled 5 m m or more. Component orientation was neutral if the center lines of the c o m p o n e n t and femur were within 3°; otherwise, the c o m p o n e n t was designated as either varus or valgus. All changes a r o u n d the cementless femoral comp o n e n t were d o c u m e n t e d using a system modified from that suggested by Engh et al. [12-14]. The femoral interface was divided in two zones. Zone I was defined as the area a r o u n d the porous surface of the femoral component; zone II was distal a r o u n d the smooth part of the stem [15]. Calcar changes (atrophy or hypertrophy), radiolucent lines, endosteal n e w bone formation near the prosthesis (spot welds), presence of a distal pedestal (an intramedullary shelf of n e w bone at the distal tip of the prosthesis), and cancellous bone changes were

evaluated according to definitions described by Engh et al. [14]. Proximity of the c o m p o n e n t to within 1 m m of endosteal bone was identified in each zone, and anteroposterior fill of the medullary bone by the c o m p o n e n t was measured at the junction of zones I and II and expressed as a percentage. The primary sign of c o m p o n e n t instability was subsidence of 5 m m or greater; conversely, the absence of subsidence was considered a sign of probable c o m p o n e n t stability. Other signs suggestive of instability were calcar hypertrophy, distal pedestal formation, and the development of progressively widening radiolucent lines a r o u n d the c o m p o n e n t [16]. Complete radiolucencies in the porous-coated area (zone I) were considered signs of failure of bony ingrowth (ie, fibrous fixation). Probable failure of ingrowth (probable fibrous fixation) was likely if the radiolucency encompassed more t h a n 50% of the interface on zone I. The presence of spot welds in zone I denoted fixation of the c o m p o n e n t by bone ingrowth [14]. Representative radiographs of an Integral femoral stem, 10 years after surgery, with a revised acetabular cup, 6 years postrevision, are shown in Figures 2 and 3.

Fig. 2. Anteroposterior view of lO-year postoperative Integral femoral stem with a 6-year postoperative acetabular cup revision. Large arrows, distal endosteal bone growth; small arrows, proximal spot welds.

Integral Porous Femoral Stem •

Fig. 3. Frog-leg lateral view of 10-year postoperative Integral femoral stem with a 6-year postoperative acetabular cup revision. Arrows, distal endosteal bone growth.

Statistics Comparisons between pre- and postoperative variables were performed using the two-tailed, paired ttest. Differences were considered to be significant at P < .05.

Results The average preoperative pain score was 19.9 (range, 0-40), increasing to 41.8 (range, 20-44) at 1 year (P < .0001) and 41.2 (range, 10-44) at the most recent follow-up visit (P < .0001). The average preoperative function score was 55.6 (range, 12-84), increasing to 92.3 (range, 58-100) at 1 year (P < .0001), and 93.5 (range, 6 - I 0 0 ) at the most recent follow-up visit (P < .0001) (Table 2). Excellent and good results were obtained in 139

Table 2. Clinical R a t i n g s of the I n t e g r a l S t e m Harris Hip Score

Pain Function

Preoperative

1 Year

Most Recent*

19.9 55.6

41.8 92.3

41.2 93.5

*Mean follow-up period was 5.8 years; range, 5-8 years.

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hips (94%). Nine hips (6%) had fair or poor results; however, of those hips, 4 hips (3%) were rated just prior to revision surgery of a failed acetabular c o m p o n e n t . At the most recent followup visit, the incidence of thigh pain was 4%. In these patients the thigh pain was present after long periods of activity and was disabling in none. Thirty-seven hips (25%) had a revision of the acetabular cup. Thirty-two (22%) that were revised for acetabular cup loosening had threaded acetabular components and 5 hips (3%) had a porous hemispherical cup with adjuvant screws. The use of 28 and 32-mm titanium femoral heads as well as thin polyethylene liners with first-generation locking mechanisms contributed significantly to these failures. In this series, the average time to acetabular failure was 5.3 years (range, 2-8 years). Of the 37 acetabular failures, 21 of 37 (57%) had an element of acetabular osteolysis contributing to failure. At the time of acetabular revision, no femoral component showed radiographic evidence of loosening, and intraoperative evaluation of the femoral component found it to be solidly fixed and stable. The return of the pain and function scores to excellent and good results after acetabular revision was reflected in the most recent clinical evaluation. Deteriorating clinical scores during the follow-up period represented impending acetabular failure. To date, none of the femoral components have been revised or showed signs of aseptic loosening.

Radiographic Analysis There were 46 hips (31%) with a Dorr bone type A, 98 hips (66%) with a Dorr bone type B, and 4 hips (3%) with a Dorr bone type C. Femoral subsidence of 5 m m or more was seen in 2 hips (1.4%); however, these two subsidence events were 5 m m and both demonstrated spot welds and no distal pedestals. In the porous-coated zone (zone I), spot welds were present in 147 (99.3%). Only 2 (1.4%) had radiolucencies that constituted greater than 50% of the porous-coated zone. These two hips probably had fibrous fixation. Distal pedestals could be identified in 13 (9.0%). Direct bone apposition in this zone was apparent. Distal cortical h y p e r t r o p h y in zone II was seen in 42 (28.6%). Calcar atrophy was absent in 32 hips (22%), present above the lesser trochanter in 96 hips (65%), and to the level of the lesser trochanter in 13 hips (13 %). Radiographic evaluation demonstrated no evidence of aseptic loosening. Measurable osteolytic defects were seen in only two patients. One patient developed a defect in the greater trochanter at 6 years. This patient subse-

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quently had a revision of a failed threaded acetabular component and grafting of a 2 x 3-cm defect. The second patient developed a defect in the region of the calcar and lesser trochanter at 6 years. This patient also had a revision of a threaded acetabular component without grafting of the calcar defect. There were no cases of distal osteolysis at the time of this report. The anteroposterior fill ratio had a m e a n of 0.81 (range, 0.54-0.95). On the a n t e r o p o s t e r i o r radiograph, 94.6% of patients w e r e judged to have an excellent fit. On the lateral radiograph, only 67.3% were judged to have an excellent fit. There was no correlation b e t w e e n fit and fill and radiographic c o m p o n e n t migration or change in varus or valgus position. Additionally, there was no correlation b e t w e e n fit and fill, Dorr b o n e type, and clinical outcome.

Discussion Cenrentless fixation of the femoral c o m p o n e n t in THA can be achieved with stems of different geometry, metallurgy, porous coatings, and surface treatments. The degree to which each of these variables contributes to initial fixation and long-term durability continues to be elucidated as intermediate studies are published. Studies have attempted to define the ideal stem geometry for cementless fixation [17]. Others have explored the nature of porous coatings [i8]. Still others have espoused the benefits of bioactive coatings such as hydroxyapatite [ 19-21 ]. Certainly, long-term studies will clarify which variables promote the durability of THA as well as those that are detrimental, such as titanium femoral heads [22] and thin polyethylene liners [23-26]. If long-term studies show equivalency of these variables, then issues such as cost and ease and reproducibility of implantation will assume a major role in decisions of implant choice and fixation method. This study demonstrates several points. First, the femoral c o m p o n e n t design used, a tapered wedge with a proximal plasma spray coating, allowed excellent initial implant stability, with 98% radiographic bone ingrowth at 5 to 8 years. The two stems with radiographic fibrous fixation had no progressive changes and excellent clinical function. Secondly, titanium plasma spray surface treatment facilitates osseointegration of the implant radiographically in a predictable fashion. Although difficult to quantitate, endosteal bone apposition can be demonstrated in zone II of the series on both the anteroposterior and lateral radiographs. This adaptive remodeling combined with a 99% incidence of

spot welds seen in zone I (the porous c o a t i n g ) without incidence of deterioration of the bone-prosthesis interface--is an encouraging finding. Third, it is conceivable that the absence of distal osteolysis is related to the circumferential titanium plasma spray coating that may serve to inhibit distal transfer of particulate debris. Even though we did not observe distal osteolysis in this series, however, we do agree that caution must be exercised w h e n predicting h o w the bone-prosthesis interface will change over longer periods, given the constant assault of particulate debris. The presence of osteolytic defects in the calcar region, particularly under the collar, may be underestimated in this study. In our series, it is c o m m o n to find a small (< 1 cm) scalloped lesion in the region of the lesser trochanter at the time of revision of failed acetabular components. These lesions are clearly demarcated by a sclerotic bony rim that is firmly attached to the prosthesis, without distal extension. This study emphasizes the n e e d for c o n t i n u e d periodic follow-up evaluation in THA. There was a 25% acetabular failure rate, and it is anticipated that this will increase with time as more cases of asymptomatic and symptomatic periacitabular osteolysis are discovered. After an initial excellent and good period of function, we found the deterioration of clinical scores in this series was uniformly attributable to impending acetabular failure at an average time of 5.3 years from initial implantation. Femoral c o m p o n e n t s in this series all s h o w e d evidence of osseointegration and stable fixation which was maintained t h r o u g h o u t the period of acetabular failure and eventual revision. Because radiographic acetabular failure preceded clinical failure in m a n y cases, the need for continued long-term follow-up evaluations of all patients is necessary, a fact that should be continually expressed to patients, third-party payors, and health policy analysts.

Conclusion We believe these results demonstrate that a femoral c o m p o n e n t with design parameters set forth in this paper gives excellent clinical and radiographic results in the intermediate follow-up period of 5 to 8 years. The issues of durability and preservation of the bone-prosthesis interface will be further elucidated with time.

Acknowledgment We t h a n k Dr. William J. Hozack of The R o t h m a n Institute (Philadelphia, PA) for his help in the

Integral Porous Femoral Stem radiographic interpretations Integral femoral stem.

and

a n a l y s i s of t h e 13.

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