Multisurgeon assessment of surgical decision-making in adolescent idiopathic scoliosis: curve classification, operative approach, and fusion levels.

STUDY DESIGN: A multisurgeon assessment of curve classification, selection of operative approach, and fusion levels via a case study presentation. OBJECTIVES: To evaluate the ability of a group of scoliosis surgeons, not involved in the development of a new classification system, to accurately choose the corresponding curve classification of adolescent idiopathic scoliosis (AIS) cases and to evaluate the variability in the selection of operative approaches and both proximal and distal fusion levels in accordance with the new classification system in operative adolescent idiopathic scoliosis. SUMMARY OF BACKGROUND DATA: Recent evaluations using the King method for classifying AIS has shown poor intraobserver and interobserver reliability. A new, comprehensive classification system of AIS has been developed, but the result of a scoliosis surgeon's ability to apply the objective classification is unknown. In the surgical treatment of AIS, there are three choices for the operative approach (anterior, posterior, or both) and multiple choices for the selection of fusion levels. METHODS: During an AIS roundtable discussion at a spinal surgery meeting, 28 scoliosis surgeons were presented seven cases of operative AIS via good quality slides. Standard preoperative radiographs and clinical photographs were presented, and the reviewers were asked to classify the cases by a new classification system, choose their preferred surgical approach, and classify both proximal and distal fusion levels. RESULTS: For the seven cases presented, 84% of the curve types, 86% of lumbar modifiers, and 90% of sagittal thoracic modifiers were classified by the reviewers as described in the new classification. The case study found widely variable operative approaches and fusion levels chosen by the reviewers. There was an average of five different proximal (range, 4-8) and four different distal (range, 3-5) fusion levels chosen by the reviewers for each case. CONCLUSIONS: This case study assessment found a relatively high rate (84-90%) of agreement in curve classification of the individual components of a new classification system of AIS. This suggests the ability of a group of scoliosis surgeons to identify the specific criteria necessary for this new classification system of AIS. In addition, the high variability in selection of both operative approach and fusion levels confirms the current lack of standardized treatment paradigms. This further reinforces the need for a method to critically and objectively evaluate these variable treatments to determine the "best" radiographic and clinical results.

The inverse effects of load transfer and load sharing on axial compressive stiffness.

BACKGROUND CONTEXT: The goal of spinal instrumentation is to stabilize involved motion segments while fusion occurs. Although some degree of load sharing is necessary for fusion, the ability of the instrumentation system to transfer the load may vary. PURPOSE: The purpose of this study is to formulate a mathematical relationship between load sharing and load transfer of specific spinal instrumentation systems using a well-accepted mechanical model. STUDY DESIGN/SETTING: Forty-eight American Society for Testing Materials standard ultra high molecular weight polyethylene cylinders were used as per designation F 1717-96, standard test methods for static and fatigue for spinal implant constructs in a corpectomy model. METHODS: Twenty-four spinal assemblies consisting of anterior plates, anterior rod, and posterior rods were subjected to compression bending tests using a MTS Bionix servo-hydraulic material testing apparatus. Each implant was tested in compression bending with and without the addition of a titanium load-sharing cage. The force applied was the independent variable, and the displacement was the dependent variable. The stiffness was determined for each setup with and without the addition of an anterior load-sharing cage. RESULTS: The average axial compressive stiffness of a system increased by a factor of 8.5 with the addition of the load-sharing cage. An inverse relationship existed between the compressive stiffness of the construct and its relative increase achieved with the addition of the load-sharing cage. The compressive stiffness of the system with the addition of the load-sharing cage approached that of the anterior device itself as the system flexibility increased. The ability of instrumentation systems to load share or load transfer and their respective stiffness was determined. CONCLUSIONS: The 5-mm rod screw posterior system was compared with the 7-mm Ti posterior system with the addition of one and two devices for transverse traction (DTTs). The rods with the increased diameter had a stiffness of 1723 n/mm with one DTT and 1815 n/mm with two DTTs. The addition of an anterior cage had little effect on the stiffness of these systems. Anterior plate and screw/rod systems were analyzed and showed similar mechanical behavior to the 5-mm posterior rod/screw systems. A significant increase in stiffness was realized with the addition of an anterior cage. A means to determine the load sharing/transferring properties of a spinal instrumentation system is presented. This technique will allow the amount of load transferred from the fusion mass to the instrumentation to be predicted.

Results of the Scoliosis Research Society instrument for evaluation of surgical outcome in adolescent idiopathic scoliosis. A multicenter study of 244 patients.

STUDY DESIGN: An outcome questionnaire was constructed to evaluate patient satisfaction and performance and to discriminate among patients with adolescent idiopathic scoliosis. OBJECTIVES: To determine reliability and validity in a new quality-of-life instrument for measuring progress among scoliosis patients. SUMMARY OF BACKGROUND DATA: Meta-analysis of the surgical treatment of adolescent idiopathic scoliosis determined that a uniform assessment of outcome did not exist. In addition, patient measures of well-being as opposed to process measures (e.g., radiographs) were not consistently reported. This established the need for a standardized questionnaire to assess patient measures in conjunction with process measures. METHODS: The instrument consists of 24 questions divided into seven equally weighted domains as determined by factor analysis: pain, general self-image, postoperative self-image, general function, overall level of activity, postoperative function, and satisfaction. The questionnaire takes approximately 5 minutes to complete and is taken at predetermined time intervals. A total of 244 of patients from three different sites responded to the questionnaire. RESULTS: The reliability based on internal consistency was confirmed with a Cronbach's alpha coefficient greater than 0.6 for each domain. In addition, acceptable correlation coefficient values greater than 0.68 were obtained for each domain by the test-retest method on normal controls. Similarly; to establish validity of the questionnaire, responses of normal high school students were compared with that of the patients. Consistent differences were noted in the domains between the two groups with P < 0.003. The largest differences were in pain (control, 29.96 +/- 0.20; patient, 13.23 +/- 5.55) and general level of activity (control, 14.96 +/- 0.20; patient, 12.16 +/- 3.23). Examination of the relationship between the domains and patient satisfaction showed that pain correlates with satisfaction to the greatest degree (Pearson's correlation co-efficient, r = -0.511; P < 0.001), followed by self-image (r = 0.412; P < 0.001). CONCLUSIONS: This questionnaire addresses patient measures for evaluation of outcome in adolescent idiopathic scoliosis surgery by examining several domains. It also allows for dynamic monitoring of scoliosis patients as they become adults. This is a validated instrument with good reliability measures.

Occipital screw pullout strength. A biomechanical investigation of occipital morphology.

STUDY DESIGN: A three-group design with consistent pullout strength measures. OBJECTIVES: To determine pullout strength of three fixation types (unicortical screws, bicortical screws, wires) and to investigate their correlation with respect to occipital morphology. SUMMARY OF BACKGROUND DATA: A secured, multidirectional occipitocervical fusion requires internal fixation. Devices secured at occipital protuberance were suggested to offer the greatest pullout strength because of this region's thickness. METHODS: Twelve fresh human cadaveric occiputs were sketched with a grid delineating 21 fixation sites. Each site was drilled and hand-tapped. Four specimens were instrumented with unicortical screws on one side of the midline and bicortical screws on the other. Another four were instrumented with bicortical screws and wires, and the remaining four were instrumented with unicortical screws and wires. Two points on each specimen were secured with identical fixation to examine side-to-side symmetry. An MTS materials testing apparatus (MTS Systems Corporation, Eden Prairie, MN) was used to displace the fixators. Pullout strengths at different anatomic locations were recorded. RESULTS: The greatest pullout strength was at the occipital protuberance for all fixation types. The bicortical pullout strength was 50% greater than unicortical. The wire pullout strength was not significantly different from that of the unicortical screw (P > 0.05). Seventy-eight percent of wires broke at 1100 N. Unicortical pullout strength at occipital protuberance was comparable with that of the bicortical screw at other locations. CONCLUSIONS: Unicortical screw fixation at occipital protuberance offers acceptable pullout strength without the potential complications of bicortical screws or wire fixation.

Technique for the harvesting of human cadaveric spines.

STUDY DESIGN: One hundred twelve fresh cadaveric spines were harvested using a newly described technique. OBJECTIVES: To develop and describe a technique for the expeditious excision of intact human cadaveric spines for biomechanical testing, to educate the dissector on the health and safety issues involved in harvesting spinal specimens, and to review the present recommendations for storage and preservation of spinal segments. SUMMARY OF THE BACKGROUND DATA: As the need for biomechanical spinal research continues to expand, the demand for fresh human cadaveric vertebral specimens increases. Previous techniques for harvesting are simplistic and sparse. This technique offers a reliable and expeditious method for procurement of spinal vertebral segments of any size. METHODS: Human cadaveric spines were harvested using an adaptation of previous posterior spinal approaches. Techniques for sectioning each vertebral region were developed. Detailed description of these techniques was meticulously documented. The procured spinal segments have been used for multiple biomechanical investigations. RESULTS: The technique has been used successfully in more than 100 spinal harvests. Approximate time required is 30 minutes. The harvested segments have been reliable biomechanical specimens in many published studies. CONCLUSIONS: A new technique for the rapid extraction of human cadaveric spines has been developed. Dissectors may benefit from the recommendations offered for sectioning of each region.

Occipital morphology. An anatomic guide to internal fixation.

STUDY DESIGN: The authors present the results of an anatomic study of the human occiput to delineate appropriate screw placement sites. OBJECTIVES: Occipital bone morphologic characteristics were evaluated to determine whether significant variability exists and to determine the position of greatest bone thickness for safe and effective internal fixation. SUMMARY OF BACKGROUND DATA: New instrumentation and techniques for occipital fixation are being developed in response to concerns about occipital bone variability. Thirty cadaveric occiputs were evaluated to determine if such variability exists and the location of greatest bone thickness. Radial thickness, occipital locations, and gender differences, were determined. METHODS: Twenty-six skulls were sectioned sagittally to determine the contributions of the inner, middle, and outer tables to overall occipital thickness. The angle required to gain maximal cortical purchase was determined. Mean values and variance were analyzed statistically to determine variability and thickness. Data was plotted in three dimensions. Variability in morphologic features was minimal. RESULTS: The internal occipital protuberance-external occipital protuberance was thickest at 17.55 mm (SD = 3.18 mm) and was consistently located on the superior nuchal line 43 degrees from the horizontal skull base line. Bone thickness decreased radially from the central internal occipital protuberance position. Bone thickness above the superior nuchal line exceeded that below by 2.74 mm (P < 0.05) vertically and at the oblique positions (P < 0.05). Bone to the right of the midline was only 1 mm thicker than that to the left. Gender differences were minimal. The inner table contributed only 10% to overall occipital thickness. As occipital thickness decreased, the optimal purchase angle increased. CONCLUSIONS: Unicortical purchase at and above the superior nuchal line is warranted with a low risk of intracranial venous penetration. Internal fixation devices developed in response to occipital bone variability should be considered with respect to occipital bone thickness distributions. Attention to cervical morphologic characteristics should result in higher success rates in occipitocervical arthrodesis.

Computed tomographic evaluation of the normal adult odontoid. Implications for internal fixation.

STUDY DESIGN: Computed tomography scans of the dens were performed on patients who had no atlantoaxial pathology. OBJECTIVES: To determine whether one or two screws is optimal for fracture fixation and whether two screws can always negotiate the intramedullary odontoid cavity. SUMMARY OF BACKGROUND DATA: Fixation of Type II dens fractures traditionally has used C1-C2 posterior wiring and fusion. Two screws placed across an odontoid fracture as a method of rigid internal fixation also has been described. However, it is not known whether two screws can always negotiate the odontoid canal. METHODS: Ninety-two consecutive computerized tomography scans of the dens were performed on adults who had no atlantoaxial pathology. Measurements were taken from the scan and compared with the cross-sectional diameter of two odontoid screws. RESULTS: The critical diameter for the placement of two 3.5-mm cortical screws with tapping was 9.0 mm. This dimension was present in 95% of the patients studied. CONCLUSIONS: Correct orientation of the computerized tomography scanner is critical for accurate measurements. Two 3.5-mm screws can be used in internal fixation of Type II dens fractures in 95% of the patients if the inner cortex is tapped.