Heterotopic ossification has been reported to occur in as many as 90% of patients after total hip arthroplasty.28 Heterotopic bone formation is of clinical significance only if function is impaired because the disorder is extensive, a problem that occurs in a few of these patients.25,27,31,32 Loss of mobility in the involved hip is the primary problem in patients with severe heterotopic ossification.15
Virtually all of the focus of the numerous studies has been on the efficacy of non-steroidal antiinflammatory drugs7,16,29 and of radiation therapy3,6,9,10,20,26,34 for prophylaxis against heterotopic ossification. The primary end point or outcome assessed by these studies was the radiographic manifestation of the heterotopic bone after total hip arthroplasty, rather than the functional result. Few studies have evaluated the results of excision of the heterotopic bone.3,9,11,19,21,34 However, these reports have discussed the effect of excision as a secondary aspect of the study. The importance to functional outcome of heterotopic bone excision alone is difficult or impossible to assess from these studies because the reports have different designs, goals, and end points. There has been no study specifically addressing the question of the indications for and value of excision of ectopic bone as an isolated procedure. The purpose of this study was to evaluate the subjective and objective functional outcome of surgical heterotopic bone excision as a discrete procedure in a large group of patients with symptomatic heterotopic ossification after total hip arthroplasty. The goal is to shed light on the indications and anticipated outcome of this procedure.
MATERIALS AND METHODS
In a retrospective study, the records of hip replacements performed from 1969 to 1986 were reviewed; during that period, 13,440 primary hip replacements were performed at the authors' institution. This survey identified 57 cases in which heterotopic ossification developed and was treated with surgical excision without concurrent revision of the arthroplasty. Excluded were all patients treated with concurrent procedures, including removal of methylmethacrylate, removal of wires from the greater trochanter, wiring of greater trochanteric nonunion, open reduction of hip dislocation, and soft tissue releases, at the time of excision for heterotopic ossification.
By these criteria, four patients were excluded, which left 53 hips in 47 patients. The symptomatic heterotopic ossification had developed in seven hips after revision surgery and in the remainder after primary total hip arthroplasty. There were 36 hips in men and 17 in women. The right hip was involved in 29 cases and the left in 24. The average age of the patients was 60 years (range, 31-77 years). The mean weight was 84 kg (range, 50-104 kg), and the mean height was 167 cm (range, 115-183 cm). Six cases were bilateral, and one patient underwent a second excision of heterotopic bone on the same hip. The underlying diagnosis leading to total hip arthroplasty had been osteoarthritis in 33 hips, congenital hip dysplasia in 10, avascular necrosis in three, and various other diagnoses in seven.
The total hip arthroplasty was cemented in 50 hips and uncemented in one hip; two hips had an uncemented acetabular component and a cemented femoral component. Thirty-three hips had trochanteric osteotomy associated with the total hip arthroplasty. Sixteen patients had a history of structural hyperostosis, two of whom had ectopic bone of the knees, four of the other hip, and 10 with no other joint involvement. Four patients had a history of ankylosing spondylitis. Two of these four patients also had evidence of ectopic bone at the time. A history of heterotopic ossification around the ipsilateral hip was present in four cases, in the contralateral hip in 17, and in both hips in seven.
Preoperative Clinical Evaluation
Clinical evaluation forms are used for all patients undergoing total hip arthroplasty at the authors' institution.12 Clinical data are obtained prospectively by the operating surgeons at the preoperative evaluation and at designated intervals after surgery.
Thirty-three hips had flexion contractures before surgery. The flexion contracture was 5° in five patients, 10° in eight, 15° in four, 20° in eight, 25° in two, 30° in three, 35° in one, 40° in one, and 45° in one. This variable was not available in two patients. The mean preoperative hip flexion was 42° (range, 0°-90°; median, 38°). The mean abduction was 9.5°, mean abduction 10°, mean external rotation 9°, and mean internal rotation 4°.
The mean preoperative arc of flexion by surgical indication was 22° for decreased range of motion (ROM), 32° for decreased ROM and pain, 74° for the pain only group, and 90° for those with instability.
Before surgery. pain was rated as severe in seven patients, moderate in 14, slight in 10, and none in 20. This variable was not recorded in two patients.
Preoperative Radiographic Evaluation
All hips were graded by the Brooker et al5 and the Mayo22 classification based on anteroposterior (AP) radiographs. Radiographs were interpreted by two observers who were not blinded relative to their interpretation but were blinded with respect to treatment and clinical outcome. The preoperative Brooker class was 4 in 47 hips, 3 in five hips, and 2 in one hip. The preoperative Mayo classification was 4 in 47 hips and 3 in six hips.
The Brooker classification was compared in independent patient groups with Kruskal-Wallis tests or with Wilcoxon rank sum tests. The distribution of changes in satisfaction, pain, and ROM from before surgery to last followup were evaluated with Wilcoxon signed rank tests.
Range of motion was compared in independent patient groups using one-way analysis of variance. Changes in ROM from before surgery to after surgery were evaluated with paired Student's t-tests.
The association of the Brooker classification with ROM was assessed with Spearman's rank correlation coefficient; p values of less than 0.05 were considered statistically significant.
The primary indication for surgical excision of heterotopic ossification was decreased ROM in 32 hips, decreased ROM with pain in 14 hips, only pain in five hips, and instability thought to be attributable to the heterotopic bone in two hips.
All 53 hips were treated by surgical excision of the heterotopic bone. The median time from total hip arthroplasty to heterotopic bone excision was 392 days. Twenty-one patients had the index procedure 1 year or less after total hip arthroplasty. The mean operative time was 2 hours (range, 1-5 hours). The surgical approach was anterolateral in 28 hips, posterior in five, and transtrochanteric in 20. The mean estimated blood loss was 822 ml (range, 100-2000 ml). The mean blood replacement was 1.5 units (range, 0-5 units).
Forty-four hips received postoperative radiation therapy and nine did not. Radiation therapy of 2000 cGy was initiated by the second postoperative day in 15 hip replacements, on the third or fourth postoperative day in 11, and after the fourth postoperative day in 18. Shielding of the components was performed in all instances.
In 14 patients, nonsteroidal antiinflammatory drugs were given after surgery. However, only four patients received specific prophylaxis therapy in the form of 75 mg of indomethacin daily for heterotopic ossification. In the remaining 10 patients, various nonsteroidal antiinflammatory drugs were given for reasons other than heterotopic ossification prophylaxis. Nine additional patients received aspirin (usually 325 mg twice a day) as prophylaxis against deep venous thrombosis. Six patients received diphosphonate therapy; however, all of these patients also received radiation treatment.
Twenty-six patients had postoperative regimens of radiation therapy or radiation therapy and nonsteroidal antiinflammatory drugs initiated on or before the fourth postoperative day, whereas 27 patients had radiation therapy initiated after the fourth postoperative day or had no postoperative treatment.
Because the focus of this study was to assess specifically the functional outcome of surgical excision and because of various postoperative treatments and timing of treatment, the outcome was not assessed on the basis of the type of postoperative treatment. However, recurrence of the heterotopic ossification was documented carefully and correlated with final motion.
The mean followup for postoperative ROM was 3.5 years (range, 6 months-16.5 years) for the 47 hips for which this variable was recorded. The mean radiographic followup was 7.8 years (range, 1.2-18.2 years) for the 38 hips with radiographs at least 1 year after surgery. Two of the patients with heterotopic bone excision for instability were followed up for 3 years in one and 6 years in the other with examination and radiographs.
Radiographic Evaluation Performed Immediately After Surgery
The Brooker class immediately after surgery was Class 4 in one hip, Class 3 in 10, Class 2 in two, Class 1 in 32, and Class 0 in eight. The Mayo grade immediately after surgery was Grade 4 in one hip, Grade 3 in 10, Grade 2 in three, Grade 1 in 31, and Grade 0 in eight. The effect of surgical excision for heterotopic ossification, as determined by Brooker class, is shown in Table 1. The difference for the entire group was statistically significant (p < 0.0001, Wilcoxon signed rank test).
Radiographic Evaluation at Followup
Table 2 compares the preoperative Brooker class and the Brooker class at followup (the first available radiograph taken at or after 1 year was used for this variable). Fifteen patients did not have adequate postoperative radiographs at more than 1 year after excision for heterotopic ossification. The Brooker class among patients with satisfactory radiographs was Class 4 in 10 patients, Class 3 in 10, Class 2 in six, Class 1 in 11, and Class 0 in one. In 28 of 38 patients, the final Brooker class had improved from the preoperative classification (Fig 1). This difference was statistically significant (p < 0.0001, Wilcoxon signed rank test).
The mean preoperative arc of flexion was 31° ± 30° for the entire group (range, 0°-90°), whereas the mean arc of flexion of 44 hips with adequate documentation was 64° ± 21° (range, 20°-95°). The difference was statistically significant (p < 0.0001, paired t test). Other parameters of motion were analyzed and found to have improved significantly (p < 0.05) for the overall group. The mean abduction and adduction arc improved by 22° and the mean arc of internal and external rotation by 21° (Table 3). Significant improvement from the preoperative values was found in flexion contracture (-8°; standard deviation, 12°), maximum flexion (23°; standard deviation, 28°), abduction (14°; standard deviation, 13°), adduction (8°; standard deviation, 9°), external rotation (15°;standard deviation, 16°), internal rotation (6°; standard deviation, 12°), and pain (-0.5; standard deviation, 1.1) (all p < 0.05). When ROM was assessed as a function of time since surgery, no significant change in ROM was found in any values obtained more than 6 months after surgery (p = 0.15).
The following variables improved slightly from before to after surgery, but the changes were not statistically significant: limp, ability to climb stairs, ability to get in and out of a car, need for support, distance walking, and ability to put on socks (all p > 0.05) (Table 3). Patients were questioned concerning their subjective overall evaluation of their outcome. Their satisfaction score (numerical scale of 1 to 4, with 1 being extremely satisfied and 4 denoting dissatisfaction) was improved (from 3 to 1.6) at followup (p < 0.001).
Clinical and Radiographic Correlation
Brooker class and ROM at followup were found to correlate (Fig 2). Range of motion, as anticipated, decreased slightly as the Brooker grade increased (p < 0.02, r = -0.42) (Fig 3).
Year of Surgery
The study spanned a 20-year period, from 1970 to 1990. Because of this, most patients received no or (by today's standards) inadequate postoperative heterotopic ossification prophylaxis (including radiation therapy after the fourth postoperative day). Most patients receiving nonsteroidal antiinflammatory drugs and radiation were treated in the late 1980s. Patients who underwent treatment in the 1980s had statistically greater improvement in ROM than did those receiving treatment in the 1970s (p < 0.0001).
Nineteen patients who had pain as a sole indication or pain in conjunction with stiffness as the indication for surgery had an average preoperative pain score (score of 1 corresponding to no pain and score of 4 corresponding to severe pain) of 3.3 (moderate to severe) and an average postoperative score of 2.3 (mild to moderate) (p < 0.001, Wilcoxon signed rank test). Of the five patients who underwent surgical excision for heterotopic ossification because of pain as the only indication for surgery, three had no change in their pain score, one had an improvement of one grade, and one had a score that was one grade worse.
Two patients underwent excision for heterotopic ossification because of hip instability thought to be attributable to this disorder. In neither of these cases did the total hip arthroplasty undergo additional dislocations.
Complications included hematoma, deep venous thrombosis, superficial infection, and deep infection that required implant removal in one patient each.
Heterotopic ossification is a well documented complication after total hip arthroplasty. In a recent review of 21 studies comprising 10,038 cases of total hip arthroplasty, Shaffer30 found an overall reported incidence of 42%, ranging from 8% to 90%. Heterotopic ossification was reported to occur in 0.3% to 24% (average, 5.8%) of patients after total hip arthroplasty; however, these reports were based on radiographic, rather than clinical, findings. Most patients with heterotopic ossification after total hip arthroplasty are without pain or functional limitations.15 With the exception of Grade 4, the severity of heterotopic ossification based on two-dimensional radiographic assessment does not always correlate with the severity of clinical impairment.33
The purpose of the current study was not simply to measure radiographically the occurrence or prevention of ectopic bone after hip replacement or excision. The literature contains numerous reports on the efficacy of postoperative radiation and nonsteroidal antiinflammatory drugs for prevention of heterotopic bone formation after total hip arthroplasty. There also are numerous reports of the efficacy of adjunctive treatment after excision of the heterotopic bone. Excision without adjunctive treatment has not been effective according to most reports11,23,25 and generally is not recommended.4
The focus of this study was to address a question not previously discussed to any extent in the literature: specifically, the clinical outcome of excision of symptomatic heterotopic ossification. Although some clinical data are available in small numbers,35 this is the first large series to address the clinical results of excision for symptomatic heterotopic ossification without concomitant surgical procedures in patients after total hip arthroplasty. The authors specifically have elected to focus on the effectiveness of excision depending on the principal goal of each patient's surgery: relief of pain or improvement of function. In the past, patients have been included or excluded, and the results of treatment assessed in most studies on the basis of radiographic evidence of heterotopic ossification, rather than on functional limitations associated with this disorder. In previous series, many patients who had excision for heterotopic ossification had the procedure at the time of revision arthroplasty,11,21 so the process was asymptomatic in most. Thus, there is no information in the literature that addresses the outcome of heterotopic excision based on the surgical indications. Clinically relevant questions include to what extent excision will improve ROM, pain, and function in the patient with symptoms.
The results of the current study show that surgical excision of heterotopic ossification for hip stiffness after total hip arthroplasty successfully can restore a moderate amount of ROM in all planes in affected hips. These data corroborate and extend the findings of Warren and Brooker,35 who reported 12 patients who had excision of heterotopic ossification (two had concomitant total hip arthroplasty revision) followed by radiation and found only one failure. Preoperative ROM was not reported; however, the average gain in flexion and abduction were 45° and 25°, respectively.
The current study indicates less remarkable improvements in the ability to perform daily activities (and in overall function) than for improvement in ROM. Patient satisfaction with the procedure, perhaps the most important measure of outcome, was high.
These data suggest that in most instances pain should not be considered a primary indication for excision of heterotopic ossification. Of the five patients with pain as the sole indication for excision, only one reported improvement and none had complete relief of pain. Heterotopic bone usually does not cause pain after maturation is complete.1,2,8,14,22,31 An explanation for this finding is not clear from the current data. The authors' perception is that because most patients experience pain during the formation of ectopic bone, a type of pain behavior may develop in some. Thus, the stimulus for pain ceases, but the perception continues. In such patients, surgical excision would not be expected to be of value.
Improvements in surgical technique and better heterotopic ossification prophylaxis with nonsteroidal antiinflammatory drugs and radiation therapy probably contributed to the better results obtained for patients receiving treatment more recently.
In the current series, which spans 20 years, during which time an understanding of the value of appropriate heterotopic bone prophylaxis was accumulating, approximately 50% (26 of 53) of the patients received what now would be considered appropriate postoperative prophylaxis against heterotopic recurrence. The effectiveness of antiinflammatory agents.7,16-18,24,29 and radiation therapy3,6,9,10,13,20,26,34 in preventing heterotopic bone formation after total hip arthroplasty is well accepted. The retrospective nature and limited sample size of the current study preclude a meaningful sub-analysis of the effect of the several postoperative treatment regimens used on outcome. Nevertheless, it is thought that prompt initiation of a proven method of heterotopic bone prophylaxis is indicated after surgical heterotopic bone excision.
For all patients in this series, radiographic outcome was statistically significantly correlated with functional outcome, but this correlation was loose. Review of the surgical records of patients with Brooker Class 3 or 4 in the period immediately after surgery revealed that although a great amount of ectopic bone was left around the hip, the problematic portion had been removed, with a corresponding improvement in intraoperative motion. In addition, in patients with Brooker Class 4 at followup, the clinical function was better than expected; this result shows the limited reliability of a two-dimensional measurement to assess a three-dimensional problem.
Surgical excision of heterotopic bone performed for limited hip ROM was successful in restoring a moderate amount of motion to the affected hip in all planes. Surgical excision for heterotopic ossification rarely provided dramatic relief of hip pain. Because of the marginal inverse correlation between radiographic outcome and hip ROM, Brooker class should not be used as the only variable for assessment of outcome of excision for heterotopic ossification.
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