Eight-Year Results of Hydroxyapatite-Coated Hip Arthroplasty

The Journal of Arthroplasty Vol. 21 No. 4 2006

Eight-Year Results of Hydroxyapatite-Coated Hip Arthroplasty Je-Ken Chang, MD,*y Chung-Hwan Chen, MD,*y Kuan-Yu Huang, MD,* and Gwo-Jaw Wang, MD*y

Abstract: Ninety hips in 82 patients using Omnifit hydroxyapatite (HA)–coated prosthesis were followed for at least 7 years. All stems were stable at the final followup. However, aseptic loosening was found in 8 cups and 6 of them were revised. Two polyethylene wear were treated with inserts exchanged. The mechanical failure rate was 11.4% and the combined failure rate was 14.3% for HA-coated cup. Four other cups with wear and osteolysis without loosening or pain and 2 cups with polyethylene wear without osteolysis were still under observation. Our findings suggest that hip arthroplasties with HA coating on the smooth surface of a titanium cup is not reliable. The mid-term result of HA-coated stem is as good as that of porous-coated stem. Key words: hydroxyapatite, total hip arthroplasty. n 2006 Elsevier Inc. All rights reserved.

plain titanium press-fit or porous-coated implants after 2 years [1]. Promising results of HA-coated femoral stems were reported by several series in the short term to medium term. D’Antonio et al [3] reported 100% stability of the stems with a followup for 10 to 13 years. However, the results of the HA-coated acetabular cup varied. Geesink and Hoefnagels [4] stated that the survival rates for acetabulum components and femoral stems 6 years after implantation were 99% and 100%, respectively. Rogers et al [5] reported that the survival rate was 100% for the femoral stem and 95% for the acetabular cup at 6 years. On the contrary, Lai et al [6] reported a 31% of failure rate for cups at a 10-year follow-up. Blacha [7] reported an incidence of 31% of acetabular cup loosening after 9 years with the HA-coated ABG cup. Capello [8] reported a 10% failure rate of cups with HA coating compared with a 3% failure rate of porous-coated cups with identical geometry at 5-year follow-ups and an 11% failure rate of the HA-coated cups at 8 years [9,10]. To clarify the medium-term results for cups and stems of HA-coated hip arthroplasties, a clinical study was carried out to evaluate the clinical and radiographic results in 90 HA-coated hip prostheses with an average of 106 months of complete follow-ups.

Cemented hip arthroplasty has been very successful in the short-term and intermediate-term results. However, there are significant problems with failure after long-term use, especially in younger, active patients or heavy-duty men, due to aseptic loosening. Biologic fixations of hip prostheses were developed in response to cement fragmentation and osteolysis. Efforts to increase bone ingrowth of cementless prostheses have led to the application of hydroxyapatite (HA) coating, a biocompatible osteoconductive calcium phosphate ceramic, to the metal surface [1]. Animal studies revealed that HA coating increases the pullout strength of an implant [2]. In experimental studies, HA-coated hip implants possessed better bone ongrowth than

From the *Department of Orthopedics, Kaohsiung Medical University, Kaohsiung, Taiwan; and yDepartment of Orthopedic Research Center, Kaohsiung Medical University, Kaohsiung, Taiwan. Submitted November 26, 2004; accepted April 3, 2005. No benefits or funds were received in support of the study. Reprint requests: Gwo-Jaw Wang, MD, Department of Orthopedics, Kaohsiung Medical University, No 100 Tzu-You 1st Road, Kaohsiung City 807, Taiwan. n 2006 Elsevier Inc. All rights reserved. 0883-5403/06/1906-0004$32.00/0 doi:10.1016/j.arth.2005.04.043

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542 The Journal of Arthroplasty Vol. 21 No. 4 June 2006 Postoperatively, patients underwent 6-week toetouching weight bearing with crutches or a walker, and full-weight bearing was not allowed until 12 weeks later. The duration of protective weight bearing may be prolonged for an additional 6 weeks in revisional patients. Radiographic and clinical assessments using modified Harris hip score were collected in a prospective manner preoperatively, at 3 and 6 months, 1 year, and annually thereafter. Radiographs of the stem were evaluated by using Gruen zones [11]. Endosteal condensation (spot welds), reactive line formation, stress shielding effect (calcar round-off, osteoporosis), cortical hypertrophy, tip stability, pedestal, subsidence, and ectopic bone formation described by Engh et al [12] were recorded. A stem was defined stable with osseous ongrowth if there was an absence of radiodense lines and accretion of endosteal bone (spot welds) in the HA-coated areas. A stem was considered stable with fibrous ingrowth if there were parallel radiodense lines involving the HA-coated zones without subsidence. On the contrary, subsidence or nonparallel radiodense line around the stem will be defined as unstable [12]. New bone formation (cancellous bone condensation) and reactive line formation were recorded around the acetabular cup evaluated by methods proposed by DeLee and Charnley [13]. Progressive widening of radiolucent line more than 2 mm around the cup or migration of the acetabular component more than 5 mm or 58 was defined as loosening. Osteolysis was recorded when intense and progressive loss of bone density locally, scalloped erosion, or linear lesion of bone was identified. Marked polyethylene wear was recorded whenever the wear was more than 5 mm. The combined rate of failure was based on the number of components that had failed mechanically and the number that had been revised because of osteolysis or disabling pain.

Materials and Methods A prospected serial clinical study was carried out using Omnifit-HA hip system (Osteonics, Allendale, NJ). From March 1993 to May 1997, 102 HAcoated hip prostheses were implanted in 90 patients at our institution. Five patients died or became disabled within 5 years postoperatively for reasons unrelated to the operation and were excluded from the study. Four of them received bipolar hemiarthroplasty for senile femoral neck fracture and the other one with cup revision died 3 years postoperatively because of liver cirrhosis. Another 3 patients were lost to follow-up because of address change so that they can not be connected. Ninety hips in 82 patients were followed up for at least 7 years (average, 8 years 10 months) including 47 primary total hip arthroplasties (group 1), 15 revisional total hip arthroplasties (group 2), 8 cup revisional arthroplasties (group 3), and 20 bipolar hemiarthroplasties (group 4). In total, there were 82 HA-coated femoral stems and 70 HAcoated cups available for evaluation. The average age at operation was 54 years (range, 22-81 years): 51 years in group 1, 52 years in group 2, 50 years in group 3, and 64 years in group 4. There were 36 male and 46 female patients in our study. The details of diagnosis and treatments were listed in Table 1. All operations were performed through posterolateral approach. The Omnifit-HA hip system has HA coating on the smooth surface with macrotexture of dual geometry cup and the proximal 40% of femoral stems. The thickness of HA coating on both components was 50 lm, with 95% purity and Ca/ p ratio of 1.67:1. Initial mechanical stability of cup was achieved by press-fit with 1-mm underreaming. Enhancement of cup fixation with screws, between 2 and 4, was used in most cases. Chip bone graft (auto or allograft) was used for patients with poor bone stock in most revision cases.

Table 1. Diagnosis and Treatment in the Patients Diagnosis AVN OA Femoral neck fracture THA cup loosening Cup migration of bipolar hemiarthroplasty RA AS THA infection Total

Hip

THA

28 18 14 13 4 5 2 6 90

22 18

Revisional THA

Cup revisional arthroplasty

Bipolar hemiarthroplasty 6 14

9

4 4

5 2 47

6 15

8

20

THA indicates total hip arthroplasty; AVN, avascular necrosis; OA, osteoarthritis; RA, rheumatoid arthritis; AS, ankylosing spondylitis.

Results of Hydroxyapatite Hip Arthroplasty ! Chang et al 543

Statistics The differences in clinical results between groups were compared by 1-way analysis of variance and Scheffe´ method. The survivorship of cups was analyzed by Kaplan-Meier survivorship curve and compared by log-rank test. Significant was defined as P b .05.

Results All patients gained significant clinical improvement after operation. Six cups in 6 patients were revised because of aseptic loosening and two inserts in one patient were exchanged secondary to marked wear. At the latest follow-up, 82 hips in 75 patients survived. Clinical Results The mean Harris hip scores improved from 38.5 preoperatively to 91.2 at the third month, 91.7 at the sixth month, 93.5 at the first year, 96.9 at the third year, and 91.5 at the latest follow-up. The mean preoperative Harris hip scores were 36, 39, and 43 for groups 1, 2, and 3, respectively. At the third month, the Harris hip scores were 92, 89, and 93 for groups 1, 2, and 3, respectively. At the sixth month, the Harris hip scores were 94, 89, and 92; at the first year, the scores were 94, 93, and 93; at the third year, the scores were 97, 96, and 96 for groups 1, 2, and 3 respectively. At the final followup, the scores were 93, 90, and 90 for the different groups. No significant differences were noted by analysis of variance. In group 4, bipolar hemiarthroplasties included 14 aged patients because of senile femoral neck fracture. The clinical results of group 4 were not shown for comparison.

ongrowth at zone I or III was noted at 2 years. Aseptic loosening with cup migration was found in 8 cups at 15, 21, 32, 57, 71, 98, 116, and 120 months postoperatively and 6 of them had been revised. The remaining 2 loosed cups were under observation because the symptoms were mild and the patients did not want to receive reoperation at the present time. The times to loosening were 15, 32, 98, and 116 months in group 1; 57 and 120 months in group 2; and 21 and 71 months in group 3. Marked insert wear without cup loosening was noted in 8 hips. Two were treated with insert exchange in a young male because of marked osteolysis 8 years after operation. Four cups with insert wearing and some osteolytic lesions at retroacetabular area and another 2 without obvious osteolysis were still under observation. The mechanical rate of failure is 11.4% (8/70) and the combined rate of failure is 14.3% (10/70). Complications There was no mortality within 1 year postoperatively and no deep vein thrombosis, infection, or neurovascular injury in our series. Six intraoperative calcar splits treated with immediate wiring united uneventfully without affecting stem stability. There were 3 hip dislocations and all 3 dislocations happened accidentally due to inappropriate posture at 24, 28, and 34 months after operation, respectively. All dislocations were successfully treated by close reduction and there were no redislocations. Thigh pain was complained by 4 patients including 2 with severe spondylosis and typical radicular pain along the lateral thigh to the peroneal area. There was no intraoperative complication in these 4 cases and all x-rays showed proper prosthesis size and position. There were 2 hips with mild heterotopic ossification (Brook I). The ranges of

Radiological Results All 82 stems were inserted within 58 varus or valgus position. In more than half of the stems, bone ongrowth was seen around the HA coating of the implant at 6 months; it became more prominent in most cases after 2 years. Evidences of the stress shielding effect, calcar round-off and osteoporosis, were obvious after 3 months and appeared in all patients 2 years after surgery. Although two pedestal formations were found, all stems were stable with osseous ongrowth and there was no subsidence observed. For all cups, the inclination was in the range of 358 to 558. In the acetabulum, bony remodeling around the cups developed more slowly. Bone

Table 2. Details of the Loosening Cups in 8 Patients

Case

Sex

Age

Side

Diagnosis

1 2

F F

64 58

R R

3 4

M F

71 45

R L

5 6 7 8

F M M F

34 40 48 69

R L R L

AVN THA cup loosening OA THA infection SLE AVN OA THA cup loosening

Time after implantation (mo)

Status

15 21

Revised Revised

32 57

Revised Revised

71 98 116 120

Observed Revised Observed Revised

544 The Journal of Arthroplasty Vol. 21 No. 4 June 2006

Fig. 1. Kaplan-Meier survivorship curve estimated for the combined rate of failure as an end point. The predicted survival rate of cup is 69%, including 69% for group 1, 74% for group 2, and 75% for group 3. The result of log-rank test shows no significant differences between groups (v 2 = 2.2663, P = .322).

motion in these 2 hips were not affected. The detailed data of 8 aseptic loosening cups that developed within 15 to 120 months are listed on Table 2. Fig. 1 shows the Kaplan-Meier survivorship curve of the cup. With cup revision, insert exchange, and aseptic loosening as the end points, the predicted survival rate by survivorship analysis is about 69% after 10 years in our series, 69% for group 1, 74% for group 2 at 10 years, and 75% for group 3 at 9 years. No significant differences were noted between by log-rank test ( P = .322).

Discussion This study reports the mid-term clinical and radiographic results of HA-coated hip prostheses. In general, our results were comparable with other reported series [4,14-16]. We used the HA-coated hip prostheses not only for the primary hip arthroplasties, but also in the revision arthroplasties. Twenty-three (33%) of 70 cups and 15 (18%) of 82 stems were revision arthroplasties, a higher indication proportion as compared with other studies [15]. Theoretically, the osteoconducting properties of HA coating may offer a greater chance of early osteointegration. Soballe et al [17], using roentgen stereophotogrammetric analysis, confirmed that HA coating was superior to titanium porous coating in the fixation of the femoral stem. Maruyama et al [18] used the HA

clay to fill the gap between uncemented implant and the host bone with adequate bone ingrowth in animal study. A wide gap would be usually produced at the revision arthroplasty. The use of HA-coated implants with chip bone graft in revision hip arthroplasty would gain the advantage over the earlier biologic fixation. There was slight difference of Harris hip scores between primary and revision cases early after the operation, but the scores of revision cases were close to those of primary cases at 1 year. Radiographically, new bone formation usually developed to fill the gaps in revision cases by 3 months. With respect to thigh pain, our incident rate was low. If we excluded two patients who had severe spondylosis with typical radicular pain, the thigh pain occurred only in 2 of 82 stems. There was no correlation with intraoperative cracks or inappropriate size in these two cases. Low incidence of thigh pain in HA-coated hip prostheses was also reported by Geesink and Hoefnagels [4], Rogers et al [5], and D’Antonio et al [3]. However, Tonino et al [16] reported the virtual disappearance of any form of disabling pain after 3 months postoperatively, but some form of thigh pain appeared in 3.6% of patients after 2 years. According to the radiographic criteria for stability and fixation described by Engh et al [12] and Geesink and Hoefnagels [4], all our femoral stems had secured bone ongrowth. Because the press-fit technique was used for the stem insertion, one

Results of Hydroxyapatite Hip Arthroplasty ! Chang et al 545

third of femoral stems were seen to have calcar round-off at 3 months due to proximal fixation and no gaps were seen around the HA-coated area. We think that true biologic fixation of femoral stems would not happen until formation of spot welds and further signs of bone remodeling appeared. In most cases, cortical hypertrophies just distal to the transitional zones were seen after the well development of spot welds around the proximal femur. There were signs of stress shielding, which was represented by calcar round-off and osteoporosis of proximal femur. Bone reactive lines occurred often in the distal part, the noncoated area, of the stem and some around the shoulder of femoral stem. This sign reflected the relative micromotion between the smooth and the surrounding bone due to different Young modulus. We found that these bone reactive lines were found in about 90% of stems at 3 years. In contrast to the general good results of HAcoated stem reported [3-5,19], the results of HAcoated cup varied greatly in different series. The cup survival rate ranged from 65% to 99% at 5 to 10 years of follow-up [4-10,20-22]. It is very hard to conclude the effect of HA-coated cup in the published series because of the different designs and manufactures. Our results of HA-coated cup with 11.4% loosening rate were similar to those results with the same implant, smooth surface Omnifit HA-coated cup [9,10,23]. The time to cup loosening varied from 15 to 120 months. There were 8 other cups; 11.4% with marked insert wear with or without osteolysis. Although these cups are still stable radiologically, the results of longer follow-up would be pessimistic. Two of the cups had been treated with insert exchange at 6 and 8 years after hip arthroplasty. With cup revision, insert exchange, and aseptic loosening as the end points, the predicted survival rate by survivorship analysis is about 69% at the end of the 10th year. Unacceptable high rates of failure in Omnifit HAcoated cups were noted in our series. In our retrieved cups, most of the HA coating in the cup was lost. Manley mentioned higher rates of revision associated with the HA-coated press-fit cups with smooth metal surface, suggesting that the expected fixation of many of these implants was not achieved. Physiological loads will produce very high local shear stresses at the fixation interface. Such shear stress could lead to failure of fixation by HA coating [24]. Mann [25] reported that if a socket was not interlocked but only bonded to the surface of the acetabulum, the shear stresses at the periphery of the fixation interface were 10 times greater than they would be if the metal

shell was physically interlocked with the acetabulum. Because the HA coating over the smooth surface of the cups was not physically interlocked, only bonded to the surface of the acetabular component, we agree with Chung et al [21] that the failure of the cups with HA-coating on the smooth-surface cup will not maintain stability and eventually fail without osteointegration. After the HA coating has been absorbed, the macrotexture of the metal surface might be superior to the smooth metal surface because of stronger interlocking at bone-metal interface. In conclusion, the clinical and radiographic results of our 82 HA-coated stems were satisfactory with mean 106 months of follow-up. However, the results of Omnifit acetabular cup with HA coating on the smooth titanium surface were less favorable with high rate of loosening and insert wearing. Newer design with the HA coating on the rough or porous surface of titanium cup would theoretically improve the cup stability. However, the long-term results are still not available. Besides, the reason of high rate of insert wearing needs to be elucidated and corrected.

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