Heterotopic ossification after uncemented hydroxyapatite-coated primary total hip arthroplasty

The Journal of Arthroplasty Vol. 16 No. 8 2001

Heterotopic Ossification After Uncemented Hydroxyapatite-Coated Primary Total Hip Arthroplasty Ravikumar J. Kasetti, Mch Orth, FRCS, FRCS Orth, A. A. Shetty, Mch Orth, FRCS, and C. Rand, FRCS

Abstract: A total of 134 patients who had a cementless hydroxyapatite-coated total hip arthroplasty (THA) and had no recognized risk factors for heterotopic ossification (HO) were reviewed retrospectively. The average follow-up period was 83 months. Pharmacologic or radiotherapeutic prophylaxis against HO was not used. HO was seen in 90 (67.2%) of the 134 patients reviewed. HO was rated as Brooker class I in 68 (50.7%) patients, class II in 17 (12.7%) patients, class III in 3 (2.2%) patients, and class IV in 2 (1.5%) patients. The average Merle d’ Aubigne score in patients with mild forms of HO (Brooker classes I and II) was 16 compared with 13.2 in patients with severe HO (Brooker classes III and IV). A statistically significant negative correlation was found between the presence of HO and the postoperative hip score. Based on our study findings and on a comparison with data in the literature, hydroxyapatite-coated cementless THA alone does not seem to increase the likelihood of HO over other types of THA. The presence of a hydroxyapatite coating does not result in more class III or IV HO, which are the only classes of HO with real clinical significance. Fear of HO should not be a factor in the choice of fixation for THA. Key words: hydroxyapatite, total hip arthroplasty, heterotopic ossification.

Heterotopic ossification (HO) after total hip arthroplasty (THA) is a well-recognized and an intriguing complication. Highly differentiated bone forms at a place where bone was not meant to be. The radiologic incidence of HO after cemented THA in patients who have not had any prophylactic treatment has been reported widely and varies from 12% to 63% [1– 6]. The incidence of HO resulting from a

cementless THA is reported to vary from 26% to 80% [22,23]. This wide range seems to indicate not only a variation in the incidence, but also that some researchers tend to record only large amounts of heterotopic bone, whereas smaller amounts may have been overlooked or disregarded. For proper evaluation, it is necessary to compare immediate postoperative radiographs with radiographs taken later to avoid the misinterpretation of remaining bone or exostoses as heterotopic. Hedley et al [7] reported a 43% incidence of HO after 118 Porous Coated Anatomic (PCA) THAs (Howmedica Inc., Rutherford, NJ). Rockwood and Horne [8] reported an incidence of 26% after 196 consecutive RM isoelastic cementless THAs (Mathys Medical Ltd., Bettlach, Switzerland) in patients who did not receive any HO prophylaxis. Callaghan et al [9] reported an 80% incidence with the PCA hip.

From the Medway Maritime Hospital, Gillingham, Kent, United Kingdom. Submitted November 10, 2000; accepted April 12, 2001. No benefits or funds were received in support of this study. Reprint requests: Ravikumar J. Kasetti, Mch Orth, FRCS, FRCS Orth, 15 Springwood Close, Maidstone, Kent ME16, 9PA, UK. Copyright © 2001 by Churchill Livingstone威 0883-5403/01/1608-0014$35.00/0 doi:10.1054/arth.2001.25550

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Heterotopic Ossification and THA • Kasetti et al.

Hydroxyapatite-coated THA has been in clinical use since the 1980s. The incidence and effect of HO in uncemented hydroxyapatite-coated total hip systems have received little attention in the orthopaedic literature. The present retrospective study evaluated the incidence and severity of HO in THA using an uncemented hydroxyapatite-coated prosthesis.

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(JRI Ltd, London) was implanted using the WatsonJones anterolateral approach in all patients. The acetabular component and the femoral stem were coated fully with hydroxyapatite of 200 ␮ thickness. In 10 patients, femoral head autograft was used to augment the deficient acetabulum. No patient received pharmacologic or radiotherapeutic prophylaxis against HO. All patients received prophylaxis against deep vein thrombosis (heparin, support stockings). All patients were mobilized in the immediate postoperative period with full weight bearing. Surgical drains were left in situ for 48 hours postoperatively. Preoperative, immediate postoperative, and final follow-up radiographs were reviewed for all patients. Radiographs were graded for the presence of HO according to the following criteria of Brooker et al [10]:

Patients and Methods Between 1989 and 1992, 151 consecutive patients had a cementless hydroxyapatite-coated prosthesis implanted as a primary THA. None of these patients had a recognized risk factor for HO. By the time of final follow-up, 17 patients were excluded from the study because they were dead or had a recognized risk factor for HO after THA. The risk factors included ankylosing spondylitis, juvenile chronic arthritis, diffuse idiopathic skeletal hyperostosis, Paget’s disease; previous surgery on the same hip, hypertrophic osteoarthritis (osteophytes ⬎1 cm in greatest dimension), and previous ectopic bone formation. The follow-up for the remaining 134 patients averaged 83 months (range, 62–108 months). Male-to-female ratio was 1:2.5. The average age of the patients at the time of operation was 66.8 years (range, 26 – 84 years). Preoperative diagnosis was primary osteoarthritis in 125 patients (93%), osteoarthritis secondary to hip dysplasia in 4 patients (3%), osteoarthritis secondary to slipped upper femoral epiphysis in 1 patient, post-traumatic arthritis in 3 patients (2.2%), and rheumatoid arthritis in 1 patient. Using Charnley’s system of grading, 72 patients were in category A before surgery (unilateral hip arthritis), 43 patients were in category B (bilateral hip arthritis), and 19 patients were in category C (multiple joint disease). One surgeon in 1 hospital performed all of the operations. The Furlong HAC total hip prosthesis

Class I—islands of bone within soft tissues about the hip Class II— bone spurs from the pelvis or proximal end of the femur, leaving at least 1 cm between opposing bone surfaces Class III— bone spurs from the pelvis or proximal end of the femur, reducing the space between opposing bone surfaces to ⬍1 cm Class IV—apparent bone ankylosis of the hip Preoperative and the postoperative hip scores (at the final follow-up) were calculated using the Merle D’ Aubigne scoring system [11]. The data were analyzed by an independent statistician. Multiple linear regression analysis using SPSS statistical software package (SPSS Inc., Chicago, IL) was performed to look at the effect of presence of HO on patients’ postoperative Merle d’ Aubigne score, taking into account other variables such as the patients’ age, sex, Charnley grade, preoperative diagnosis, and preoperative hip score.

Table 1. Incidence, Grade of Heterotopic Ossification, and Hip Scores (Merle d’Aubigne)

Entire group Heterotopic ossification class 0 I II III IV MDA, Merle d’Aubigne.

No. Patients

Average Preoperative MDA Score

Average Postoperative MDA Score

Average Gain in MDA Score

134

7.43

15.87

8.44

44 68 17 3 2

7.55 7.41 7.29 7 7.5

15.93 16.26 14.94 16 9

8.39 8.85 7.64 9 1.5

1040 The Journal of Arthroplasty Vol. 16 No. 8 December 2001 Table 2. Heterotopic Ossification and Patient Gender

Entire group Heterotopic ossification class 0 I II III IV

No. Patients

Male

Female

134

38

96

44 68 17 3 2

12 18 7 0 1

32 50 10 3 1

Results HO was seen in 90 (67.2%) of the 134 patients reviewed (Table 1). Ectopic bone was rated as class I in 68 (50.7%) patients, class II in 17 (12.7%) patients, class III in 3 (2.2%) patients, and class IV in 2 (1.5%) patients. The incidence of HO was similar in both sexes— 66.7% in women compared with 68.4% in men. The average preoperative Merle d’ Aubigne hip score for the whole group was 7.43. At the latest follow-up, the average postoperative hip score was 15.87 (maximum possible score ⫽ 18). The average gain in the hip score in these 134 patients after THA was 8.44. Severe forms of ectopic bone (Brooker classes III and IV) were seen in 3.7% of patients (4.2% in women compared with 2.6% in men) (Table 2). The average Merle d’ Aubigne score was higher in patients with mild forms of HO (Brooker classes I and II) compared with patients with severe HO (Brooker classes III and IV) (16 vs 13.2). As the severity of HO increased, patients’ postoperative pain was found to increase, contributing to a lower overall hip score in this group. The difference in the pain score between the 2 groups (class I and II vs class III and IV) was found to be statistically significant (P ⫽ .007). Presence of mild HO did not affect the mobility of the hip or the patients’ walking

ability. Table 3 shows reduced hip mobility scores and walking scores in patients with severe HO. Two patients had class IV HO. Both patients had restricted hip movements, but movement was painful only in one. Because the study group consisted mainly of osteoarthritis patients, we cannot comment on the relationship between patients’ preoperative diagnosis and the incidence of HO. The only patient in our study with rheumatoid arthritis did not develop HO. We could not find any relationship between the patient’s age and the incidence of HO. Patients who had femoral head autograft did not have an increased incidence of HO (6 of 10 [60%] patients developed HO). Multiple linear regression analysis revealed that patient age and sex did not have any influence on the postoperative hip score. A negative correlation was found between the presence of HO and postoperative Merle d’ Aubigne score (Pearson correlation coefficient, ⫺0.31853, with 95% confidence interval, ⫺0.4631 to ⫺0.1574). This correlation was statistically highly significant with a 2-tailed P value of .0002.

Discussion There are many well-recognized risk factors for the formation of HO, including ankylosing spondylitis [12], certain surgical approaches [2,13], diffuse idiopathic skeletal hyperostosis [14], and previous surgery on the same hip [10]. Hypertrophic osteoarthritis, male gender, and previous ectopic bone formation [15] are accepted risk factors. Chalmers et al [16] proposed 3 conditions must be present for the formation of ectopic bone: i) an inducing agent, ii) osteogenic precursor cells, and iii) a conducive environment for osteogenesis. Puzas et al [17] proposed a coupling factor present in all bone that elicits stem cell activity and differentiation of bone cells. This coupling factor was seen

Table 3. Severity of Heterotopic Ossification and Hip Scores (Merle d’Aubigne)

Entire group Heterotopic ossification class 0 I/II III/IV MDA, Merle d’Aubigne.

No. Patients

Average Postoperative MDA Score

Average Postoperative Pain Score

Average Postoperative Mobility Score

Average Postoperative Walking Score

134

15.87

5.78

4.73

5.4

44 85 5

15.93 16.0 13.2

5.86 5.76 5.4

4.66 4.86 3.2

5.41 5.44 4.6

Heterotopic Ossification and THA • Kasetti et al.

in bone dust from the femora of patients undergoing THA. Most uncemented systems require vigorous canal reaming to obtain primary implant stability. Seeding of bone debris in soft tissues is usually unavoidable. If this is the case, incidence of HO after uncemented THA is increased compared with a cemented prosthesis. Maloney et al [18] in a comparative study reported significantly higher rates of classes III and IV HO after uncemented THA. The role of hydroxyapatite implant fixation as a predisposing factor in the development of HO after THA has not been assessed fully. Osteoconductive agents have been added to the proximal stem to improve fixation. Hydroxyapatite-coated porous surfaces showed an increase in the rate of bone ingrowth and the strength of attachment to bone at all intervals for 52 weeks after implantation compared with that of an uncoated porous surface [19]. Enhanced bone apposition has been shown to occur in the presence of osteoporosis in which there is a gap between the implant and the bone, and when there is early micromotion after implantation [19]; these factors have had negative effects on press-fit cementless stems not coated with hydroxyapatite [20]. The 67.2% incidence of HO in our study was comparable to figures in the literature. KromannAndersen et al [21] recorded HO in 71% of patients after cemented Charnley THA. Duck and Mylod [22] reported HO in 67% of cemented and 55% of uncemented THAS. Purtill et al [23] in a comparative study did not find any difference in the incidence of HO between patients with cemented and patients with cementless THA. Our study confirms the finding of Taylor et al [24] that formation of HO was associated with pain. It is widely reported that men have a greater tendency to develop HO than women [3,15,21]. We report an equal tendency for women to develop HO. Severe forms of HO tend to cause pain, limit mobility of the hip, and accordingly mar the postoperative result. Nonsteroidal anti-inflammary drugs (NSAIDs) have been shown to inhibit clinically significant HO effectively [25,26]. NSAIDs have been shown to reduce bone ingrowth in porous implants [27] and to inhibit new bone formation in response to bone induction [28]. These findings have raised the question whether NSAIDs might increase the risk of mechanical prosthetic loosening. Incidence of severe forms of HO in our study was only 3.7%. We recommend prophylaxis to be used only in highrisk cases even if the patient is receiving an uncemented hydroxyapatite-coated THA.

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Conclusion Hydroxyapatite-coated cementless THA alone does not seem to increase the likelihood of HO over other types of THA. The presence of a hydroxyapatite coating does not result in more class III or IV HO, which are the only classes of HO with real clinical significance. Fear of HO should not be a factor in the choice of fixation for THA.

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