O THE EDITOR:
In "Slipped Capital Femoral Epiphysis. The Prevalence of Late Contralateral Slip" (78-A: 226—230, Feb. 1996), Hurley et al. looked carefully at age, gender, race, treatment modality, chronicity of the slip, and grade of the contralateral slip. The Discussion section also pointed out that slipped capital femoral epiphysis is probably always bilateral and that even a modest degree of slip can be clinically important with time.
The authors could not determine whether any variable other than the type of treatment was important. In the Discussion section, they raised important points about the altered mechanics in a patient postoperatively or a patient who wears a spica cast.
They also pointed out, as others have1,2, that patients who have slipped capital femoral epiphysis have femoral retroversion.
The authors' conclusion was that a large prospective study is necessary to examine fully the risk of development of a detectable slip of the capital femoral epiphysis. Such a study clearly is needed. In any future studies, the investigators should pay close attention to several other important variables. In addition to femoral retroversion, patients who have slipped capital femoral epiphysis have a decreased neck shaft-plate shaft angle, which markedly reduces resistance to shear stresses. Also, at my practice, the ratio of the length of the femoral neck to its diameter in seventy-two femora was 1.20 in patients who had slipped capital femoral epiphysis, compared with 1.02 in patients who did not have a slip. There is often considerable variation between sides to explain why one side slips and the other does not1,2.
Since the level of activity and body weight are important factors in the generation of shear forces at the growth plate, these variables are also critical; however, they were not addressed in the current work. They should be in any future study.
James W. Pritchett, M.D.: 1200 12th Avenue South, Seattle, Washington 98144
Dr. Hurley, Dr. Betz, Dr. Loder, Dr. Davidson, Dr. Alburger, and Dr. Steel reply:
We agree completely with Dr. Pritchett that any study addressing the risk of a contralateral slipped capital femoral epiphysis must be prospective and must include analysis of the mechanical factors that are known to have a possible effect on shear stresses. It would certainly need to include well controlled computed tomography scans in addition to a study of several clinical factors, such as postoperative level of activity and body weight, as suggested by Dr. Pritchett. We are hopeful that our article will stimulate a large-scale prospective study.
James M. Hurley, M.D.; Randal R. Betz, M.D.; Richard S. Davidson, M.D.; Howard H. Steel, M.D.: Shriners Hospital for Crippled Children, Philadelphia Unit, 8400 Roosevelt Boulevard, Philadelphia, Pennsylvania 19152
Randall T. Loder, M.D.: Section of Orthopaedic Surgery, University of Michigan Hospitals and Clinics, 1500 East Medical Center Drive, Ann Arbor, Michigan 48109
Philip D. Alburger, M.D.: Department of Orthopaedic Surgery, Saint Christopher's Hospital for Children, Front and Erie Streets, Philadelphia, Pennsylvania 19134