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.

Meta-analysis of surgical outcome in adolescent idiopathic scoliosis. A 35-year English literature review of 11,000 patients

STUDY DESIGN

Meta-analysis of the English literature on the surgical treatment of adolescent idiopathic scoliosis.

OBJECTIVE

To gather comparable data from a number of different sources and combine the data to create a larger, more statistically significant pool of information for the analysis of surgical outcome.

SUMMARY OF BACKGROUND DATA

Meta-analysis is a technique of scientific literature review used in outcome evaluation of medical treatment. This technique has been applied to the surgical outcome of adolescent idiopathic scoliosis.

METHODS

A structured literature review was performed that cross-referenced English literature articles pertaining to the surgical treatment of adolescent idiopathic scoliosis with a focus on patient-based outcomes. Measures of patient satisfaction were compared with process measures of care.

RESULTS

A number of patients (10,989) were reviewed in 139 patient populations. Unspecified curve types (9424) and King curve types (1565) were reviewed over a 35-year period from 1958 to 1993. Of the patients, 87.32% were studied retrospectively and 12.70% prospectively. Effect-weighted follow-up was 6.8 years. Only studies with complete process and patient data for unspecified or King curve types were included for satisfaction correlation calculations. Pearson product moment correlation for n = 33 studies, n' = 2926 patients revealed a positive r' = 0.628 correlation between degree of curve correction and percent satisfaction per study. To determine the degree of curvature correction resulting in patient satisfaction, a stepwise multiple linear regression analysis was performed with level of confidence (P < or = 0.05). Of significance was that the degree of curvature corrected accounted for all the satisfaction variance predicted. A significant correction exists between degree of curve correction and percent of patients satisfied. The percent of correction and the Group type (either unspecified or King classified), did not significantly alter this prediction. The best predictor of satisfaction appears to be degree of curve correction according to these data.

CONCLUSION

Patients appear to be more satisfied by the magnitude of curve correction rather than the percent of curve correction. The degree of curvature before surgery did not predict patient satisfaction. Pearsons r' = 0.045. Satisfaction appears to be best predicted by the degree of correction only and not by the percent curve correction, the curve magnitude before surgery, nor the Group type (King, unspecified). Patient satisfaction is subjective. It does not reflect the benefits of surgery with respect to the future preservation of pulmonary function in thoracic curves nor the prevention of osteoarthritis in lumbar curves.

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.

Hemodynamic responses to penetrating spinal cord injuries

Although the hemodynamic response to blunt spinal cord injury has been well described, much less is known about the responses to penetrating spinal cord injuries. In order to elucidate any differences, we reviewed the last 75 patients treated over the past 12 years with penetrating spinal cord injuries. There were 67 men and eight women; the mean age was 26.2 years (range, 15-59 years); 73 patients suffered 120 gunshot wounds; one patient was injured with an ice pick; one was stabbed twice. The offending missile causing spinal cord injury entered the neck in 24%, the thorax in 56%, and the abdomen in 20%. Nine patients (12%) were complete quadriplegics and 49 patients (65%) were complete paraplegics; 69 patients (92%) had no rectal tone; 17 patients (22%) had incomplete injuries. Despite the high proportion of complete spinal injury (78%), only 18 patients (24%) were hypotensive in the field. Five additional patients became hypotensive in the ED. Of the 23 patients with hypotension, 18 (74%) had significant blood loss to explain their low blood pressure. The mean HR was 100 beats/minute in the field (range, 50-130 beats/minute) and 90 beats/minute in the ED. Only five patients (7%) demonstrated the classic presentation of neurogenic shock (hypotension and bradycardia). This classic presentation of neurogenic shock is rare following penetrating spinal cord injury. Despite evidence of a complete spinal cord injury on initial physical examination, hypotension is usually secondary to blood loss in these patients. A careful search for sources of blood loss is mandatory before ascribing hypotension to spinal injury.