Masafumi Homma M.D.,
Glenn E. Lipton, J. Richard Bowen M.D.
Alfred I. duPont Institute
1600 Rockland Road
Wilmington, DE 19803
INTRODUCTION: The femoral head deformities in Perthe's disease are created while the necrotic femoral head is in the process of resorption or reossification. It is not clearly described how these deformities are remodeled in remodeling phase after reossification is completed. We investigated the remodeling of the femoral head after the primary healing and gave some considerations on the factors which possibly relate to the remodeling patterns of the femoral head.
PATIENTS: 355 hips of 257 patients who reached the skeletal maturity were reviewed. There were 216 boys and 41 girls. 48 patients had bilateral involvement. The age at onset was 11 months to 14 years, averaged 6 years and 5 months. The age at last follow-up was 11 to 51 years, averaged 21 years.
METHODS: Coxa magna ratio (maximum diameter of the femoral head/maximum diameter of the contralateral femoral head) was measured as an index of femoral head size in 187 hips in the patients of unilateral involvement. Mose index (maximum radius - minimum radius / maximum radius) was also measured as an index of femoral head shape in 279 hips. Both of them were measured on the x-rays at the primary healing and at the last visit. Age at onset of disease, age at primary healing, gender, Catterall classification, acetabular head index(AHI), hinge subluxation were investigated as the possible factors relating to the remodeling patterns.
RESULTS: Both coxa magna ratio and Mose index significantly improved at the last follow-up as compared with those at primary healing (p<0.0001). In coxa magna ratio, 149 hips showed improvement and 43 hips showed worsening. Among the factors investigated, the age both at onset and at primary healing in the improving group of coxa magna ratio were younger (p<0.0001) than in the worsening group. The male group also showed the better recovery (p=0.0491) in coxa magna ratio. In Mose index, 182 hips showed improvement, 69 hips showed worsening, and 24 hips had no change. Among the factors investigated, the age both at onset (p=0.0013) and at primary healing (p<0.0001) in the improving group of Mose index were younger than in the worsening group. Hinge abduction was significantly worse in worsening group of Mose index than in the improving group (p=0.0165).
DISCUSSION: Many authors pointed out that age is an important prognostic factor in Perthe's disease. This study showed the age is also a big factor for the femoral head remodeling even after primary healing of this disease. AHI, which is an index of acetabular coverage and containment of the femoral head, did not show any difference between improving and worsening group both in size and shape of the femoral head. This means that the containment is not more important than prognostic factor for femoral head remodeling. Boys remodeled better than girls but only in size. The improvement of coxa magna ratio was more sharply correlated to the age than Mose index. This may be responsible for the significant improvement of only coxa magna ratio in male, and no improvement of Mose index in male. The necrotic percent which was expressed by Catterall type did not show any difference between improving and worsening group. This means the size of necrotic area does not influence the pattern of remodeling. Hinge subluxation was 0.36 point worse in Mose worsening group than Mose improving group. This may be due to the persistence of hinge subluxation after the primary healing.
CONCLUSION:
1. Containment is not an important prognostic factor for femoral head remodeling after primary healing.
2. Age is an important prognostic factor for femoral head remodeling, especially in size.
3. Younger boys remodel better in size than girls.
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