Peter Kelleher, B.A.
William Mackenzie, M.D., Charles Scott, M.D.
Alfred I. duPont Institute
1600 Rockland Road
Wilmington, DE 19803
INTRODUCTION: Achondroplasia is the most common form of short limbed dwarfism, occurring in roughly 25 per million births in the U.S. In 1878, Parrot coined the word achondroplasia to distinguish this disorder, which is due to an intrinsic failure of proliferating cartilage in the growth plate, from other diseases of proportionate short stature such as rickets. Musculoskeletal problems frequently occur in this dysplasia. Of specific concern is the problem of genu varum and internal tibial torsion, which can result in knee instability, pain and even serious reduction in the ability to ambulate. Previous studies report an incidence of lower extremity malalignment in 15 to 35% of the population. This varus deformity is thought to be secondary to tibial bowing and fibular overgrowth has been suggested as an etiological factor. Bracing has been proposed as a method of treatment. Kopits (1975) reported the need for corrective osteotomies for symptomatic malalignment in only 17% of individuals in his population. Despite the prevalence of these problems, surprisingly little attention has been given to establishing a natural history of limb alignment or specific guidelines for surgical treatment. The object of this study is to address the natural history, analyze alignment progression with age and to investigate the effect of fibular overgrowth in genu varum.
PATIENTS AND METHODS: 72 patients with achondroplasia were seen with standing anteroposterior radiographs taken of their lower extremities. The mean age of the patients was 6 years and 2 months (ranging from 2 years and 5 months to 30 years and 5 months), with eight individuals having reached skeletal maturity by the time of their exam. The patients were placed into three age groups: 1) less than 5 years, 2) between 5 and 10, and 3) older than 10. Only one radiograph from each patient was analyzed for each age group. Because of incomplete ossification and difficulty making radiological measurements, patients under 2.5 years were excluded from the study. Eight measurements were taken on each limb for each radiograph and are demonstrated and labelled on the following slides: 1) anatomical angle - the angle of intersection between the anatomical axis of the femur and the anatomical axis of the tibia, 2) Lateral Distal Femoral Angle (LDFA) - the lateral angle between the anatomical axis of the femur and the distal articular surface of the femur, 3) Medial Proximal Tibial Angle (MPTA) - the medial angle between the anatomical axis of the tibia and the proximal articular surface of the tibia, 4) Lateral Distal Tibial Angle (LDTA) - the lateral angle between the anatomical axis of the tibia and the distal articular surface of the tibia, 5) Mechanical Axis Deviation (MAD) - the distance between the mechanical axis of the lower limb and the midpoint of the proximal articular surface of the tibia, 6) Lateral Gapping of the Knee (LGK) - the lateral angle between the articular surfaces of the distal femur and the proximal tibia, 7) Maximum length of the tibia and 8) Maximum length of the fibula.
RESULTS: Forty six of seventy two patients presented with significant varus angulation of the lower extremity (62% of the population), with an average anatomical angle across all age groups of 5 degrees. However, the location of this angular deformity differed for the various patients, with the majority of the deformity existing in the knee and tibia. Fifteen patients required surgical correction for lower limb malalignment. The average age at time of surgery was 7.5 years, with an average anatomical angle of 19 degrees prior to surgery. Two of these patients, an 8 year old male who returned 3 years after surgery and a 13 year old female who returned 6 years after surgery, underwent repeat osteotomies for correction of genu varum. Significant fibular overgrowth was seen in forty seven percent of the population, with an average tibia/fibula ratio of .96 (range of .85 to 1.15). However, there existed no statistical correlation between fibular overgrowth and any of the other alignment measurements for the lower extremities. There did exist a statistically significant change in the lower extremity alignment across the three age groups, with a spontaneous correction of over 8 degrees.
DISCUSSION: Genu varum is a common problem in patients with achondroplasia. In fact, in our patient population we noted an incidence of significant varus malalignment in 62 percent of the population, as compared to far lower rates of 15 - 35 % in previous studies. Traditionally the varus alignment seen in the lower extremities of patients with achondroplasia has been attributed to angulation of the proximal tibia. However, we now have data that demonstrate that this varus alignment has several areas of involvement: 1) distal femur, 2) knee joint, 3) proximal tibia and 4) distal tibia. While our data is not conclusive to indicate which of these levels of involvement have the most effect on varus alignment, we believe increased lateral gapping of the knee and angulation of the proximal tibia to be the most significant factors. Indications for surgical correction of genu varum in achondroplasia have not been well delineated. Current indications include increasing deformity, lateral thrusting of the knee on ambulation, excessive anatomical angle and knee pain. There is no evidence that excessive genu varum results in degenerative joint disease in adulthood. Our study suggests that some spontaneous correction is possible. We also found that there existed no statistical correlation between fibular overgrowth and varus malalignment. While this does not automatically rule out the effectiveness of fibular resection in treating genu varum, it does seriously question this technique. Fibular epiphysiodesis, if done early enough, may provide a tether to tibial growth, possibly effecting correction. In conclusion, we found that the overall varus alignment is often the sum of angular deformity at several levels. In addition, our data indicates that genu varum does not appear to be an etiological factoring genu varum. Finally, we found evidence to suggest that there is some degree of spontaneous correction of varus deformity with increasing age.
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