Brian P. Beaubien, M.S.1 ,Glenn R. Buttermann, M.D.2
- 1. Midwest Orthopaedic Research Foundation, 700 10th Ave. South, Minneapolis, MN 55415
- 2. Midwest Spine & Brain Institute, 1950 Curve Crest Boulevard, Stillwater, Minnesota 55082
The etiology of spinal deformity is unknown particularly for adolescent idiopathic scoliosis and juvenile kyphosis (Scheuermann’s kyphosis). Although the etiology for congenital scoliosis and kyphosis are well understood to be related to asymmetric growth patterns of the bony vertebra, this is not the case for idiopathic scoliosis or juvenile kyphosis which are acquired/developmental conditions. Numerous hypothesis have been presented in the etiology of idiopathic scoliosis and juvenile kyphosis; however, none of them have gained wide acceptance nor have any of them been proven.
Idiopathic scoliosis and juvenile kyphosis are not due to abnormal or asymmetric bone growth; however, it is plausible that they may be related to asymmetry of the soft tissues about the spine or possibly abnormal growth of the intervertebral disc. Some studies have found abnormalities of the intervertebral discs in both idiopathic scoliosis and Scheuermann’s kyphosis, but most authorities now believe that these changes are secondary to the deformity rather than the cause of the deformity (Taylor 1981, Oegema 1983). A few connective tissue studies have been performed which suggest that idiopathic scoliosis patients may have abnormalities of skeletal muscle or elastic fiber (Peleg 1989, Hadley-Miller 1994, Kindfater 1994).
A review of the literature reveals a paucity of biomechanical studies of spinal ligament characterization in patients who have deformity. A few studies (Chazel 1985, Dumas 1987, Goel 1986, Mykleburst 1988, Pajabi 1984, and Pintar 1992) have characterized normal values for the supraspinous and interspinous ligaments predominantly in the lumbar spine. Typically, they have revealed a stiffness of the ligaments of approximately 12 to 200 Newtons (N) per mm and a load at failure of 50 N to 180 N. Only one study has evaluated the supraspinous and interspinous ligament biomechanical characteristics in patients with deformity (Waters 1973). This study found that the elastic (Young’s) modulus of the supraspinous and interspinous ligaments in scoliosis was 129 N/mm2. However, there was no normal control group and their results are difficult to compare to other studies.
The purpose of the present study is to obtain inter- and supraspinous ligaments from patients with spinal deformity and compared their biomechanical properties, specifically analyzing the ligaments from patients with idiopathic scoliosis (who typically have a hypokyphosis of the thoracic spine) and with Scheuermann’s kyphosis (who typically have a marked hyperkyphosis of the thoracic spine). Additionally, patients who are undergoing a thoracic spinal procedure for conditions other than deformity will also have supra- and interspinous ligaments retrieved as a control group. This study hypothesis is that there will be a difference in the biomechanical characterization of these ligaments dependent upon the type of deformity and severity of kyphosis.
Thirty patients who consent to the research protocol will be studied with ten patients in each of three posterior thoracic spinal fusion groups (idiopathic scoliosis, Scheuermann’s kyphosis, and control non-deformity thoracic fusion). During the exposure for the fusion procedure, the supraspinous ligaments and intraspinous ligaments are typically discarded as waste. The intervening spinous processes are typically resected, part of which are used for bone graft although the amount of bone graft from the intervening spinous processes is minimal. In this study, four spinous processes with three intervening levels of supraspinous and interspinous ligament at the apex of the patient’s deformity would be preserved by immediate freezing upon resection rather than being discarded as waste. The spinal fusion procedure itself is in no way altered by this and thus there are no additional risks to the patient by preserving the harvested waste tissue. The collected waste tissue as described above is frozen (-20 Celsius) in a sealed container until time for biomechanical testing. Upon thawing the tissue, small drill holes will be placed into the spinous processes through which wires are then placed and attached to a connector of a MTS biomaterials testing machine. Rate controlled tension loading is applied to each bone-ligament-bone complex. A video dimensional analyzer is used to record changes in cross-sectional area of the ligament during testing. Each specimen is loaded to failure. For each individual ligament, a stress-strain curve is obtained. From this data, stiffness, Young’s Modulus, and failure loads are derived and the values are then be compared between deformity groups. The results will then be assessed as to whether they support the initial hypothesis. If differences are found, possible future histological and biochemical studies will be considered.