The influence of pedicle screw placement on thoracic trabecular strain.

STUDY DESIGN: Experimental study. INTRODUCTION: Although pedicle screw loosening and fracture are not uncommon, there is little understanding of the loading relationship between the pedicle screw and surrounding bone. There is even less understanding of the trabecular bone mechanics one a pedicle screw has been removed. OBJECTIVES: To investigate and understand the influence of the presence of pedicle screw placement and subsequent removal on vertebral trabecular strain under axial loading. SETTING: Orthopaedic Research Laboratories, University of California, Davis, USA. METHODS: Six cadaver spines were biomechanically loaded and the minimum principal and maximum shear strains were measured using texture correlation. The treatments were divided into three conditions as follows: (1) before screw placement, (2) during screw placement, and (3) after screw removal. The obtained data were statistically analyzed. RESULTS: Trabecular strain adjacent to the pedicle screw was increased following pedicle screw placement and remained high following pedicle screw removal. CONCLUSIONS: The current study demonstrates that pedicle screw placement greatly influences the trabecular bone and introduces weakness in the area following screw removal.

Salvage of a malpositioned anterior odontoid screw.

STUDY DESIGN: Description of surgical technique with case correlation. OBJECTIVE: This article presents an alternative approach to anterior odontoid screw salvage in a patient with established nonunion. SUMMARY OF BACKGROUND DATA: Type II odontoid fractures are often treated surgically because of their risk of nonunion. Anterior odontoid screw fixation offers stable fixation without loss of atlantoaxial motion. Treatment failure may occur despite adequate screw placement but is more likely when fixation is inadequate. The traditional solution is a posterior fusion. In selected cases the surgeon may want to revise the anterior instrumentation with the hope of retaining as much C1-C2 motion as possible. METHODS: A 43-year-old man presented 16 months after Type II odontoid fracture treated by anterior odontoid screw fixation. He had neck pain, instability, and a pseudarthrosis confirmed on radiographs. The screw was excessively long, piercing the C3 vertebral body and providing inadequate fixation. To avoid posterior fusion, a modified anterior approach was used. An entry point was selected 10 mm lateral to the midline, along the anterior rim of the C2 vertebral body. A large-diameter lag screw was then passed to the tip of the fragment. An angled curette was introduced into the fracture gap through the interval between the odontoid and the C1 ring. Autogenous bone was packed into the gap and along the old screw tract. RESULTS: At the 2-year follow-up the patient had a solid union with no neck pain, no headaches, no radicular symptoms, and excellent range of motion. The approach is described. CONCLUSION: In properly selected patients an anterior revision approach can provide a better outcome than posterior cervical fusion. This modified approach allows placement of an adequate fixation screw in a vertebra damaged by previous screw failure.

Spinal cord decompression: an endoscopically assisted approach for metastatic tumors.

STUDY DESIGN: The paper describes a technique for complete vertebrectomy and spinal cord decompression, followed by a formal anterior column reconstruction, using endoscopic instruments. This procedure is indicated for patients with radioresistant metastasis of the thoracic spine, particularly those involving the upper thoracic segments where a thoracotomy is difficult and carries a high morbidity, and for patients with pulmonary disease who cannot tolerate a standard thoracotomy. Results in nine consecutive cases are reported. OBJECTIVES: To demonstrate the feasibility and benefits of endoscopically assisted decompression and stabilization through a single, extrapleural, posterolateral approach. SETTING: The Cleveland Clinic, Cleveland, Ohio, USA. METHODS: Posterolateral decompression of the thoracic spinal cord offers potential advantages over traditional combined procedures (anterior thoracotomy and posterior instrumentation), including reduced operative time, decreased morbidity, and reduced hospital stay. Previous studies have not demonstrated the same neurological benefit for posterolateral decompression as for anterior vertebrectomy and decompression, however, Surgical indications, rationale and technique for an improved posterolateral approach, augmented by endoscopic methods, are provided, and initial clinical results are described. RESULTS: Drawbacks to the traditional posterolateral decompressions have included poor visualization of the spinal cord and anterior tumor, poor access to tumor on the side contralateral to the approach, and the need to manipulate the spinal cord to completely remove both adjacent tumor and tumor adherent to the dura. Transpedicular decompression using endoscopy is described in nine patients. The mean operative time for the combined procedure was 6.0 h, with a mean blood loss of 1677 cc. Neurological recovery and maintenance were excellent. Inpatient days averaged 6.5, and ICU days averaged 1.4. Two patients died of disease eight and 14 months post-op, and seven were living, with disease, 3-36 months after surgery. CONCLUSIONS: Endoscopically assisted decompression can reduce morbidity, hospitalization, and treatment costs while matching the efficacy of traditional combined procedures. Endoscopy provides a readily available and easily applied tool that dramatically improves the surgeon's vision, providing light, magnification, and a direct view of remote structures.

The effect of anterior osteophytes and flexural position on thoracic trabecular strain.

STUDY DESIGN: Compressive and shear trabecular strains were evaluated using six cadaveric thoracic spines that included anterior osteophytes. The treatments were divided into three groups: 1) osteophytes intact and the specimen in the neutral position, 2) osteophytes removed and the specimen in the neutral position, and 3) osteophytes removed and the specimen with 5 degrees of additional flexion. OBJECTIVES: To investigate the influence of osteophytes and flexural position on vertebral trabecular strain during axial compression. SUMMARY OF BACKGROUND DATA: In the thoracic spine, the incidence of anterior wedge fractures increases with the severity of kyphosis. It is unclear whether the role of anterior osteophytes in the thoracic spine is to restrict progressive kyphosis, conduct axial load anteriorly, or both. METHODS: Thoracic motion segments, T10-T12, were axially loaded in compression, and the minimum principal and maximum shear strains were measured using texture correlation. RESULTS: No dramatic changes were found in the spatial distribution of the strains following removal of the anterior osteophytes. Conversely, after removal of the osteophytes and orienting the specimen in 5 degrees of additional flexion, the strain distribution shifted anteriorly and the magnitude increased. CONCLUSIONS: This study demonstrated that osteophytes seem to restrict progressive kyphosis rather than conduct axial load anteriorly.

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out.

BACKGROUND CONTEXT: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better "fit and fill" of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. PURPOSE: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. STUDY DESIGN/SETTING: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. METHODS: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test. RESULTS: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. CONCLUSIONS: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.

Segmental instrumentation for thoracic and thoracolumbar fractures: prospective analysis of construct survival and five-year follow-up.

BACKGROUND CONTEXT: Segmental instrumentation systems have replaced nonsegmental systems in all areas of spine surgery. Construct patterns for fracture stabilization have been adapted from deformity experience and from biomechanical studies using nonsegmental systems. Few studies have been completed to validate the use of these implants in trauma or to assess their relative strengths and weaknesses. PURPOSE: To substantiate the safety and efficacy of segmental spinal instrumentation used to treat patients with unstable spinal fractures and to identify successful construct strategies and potential pitfalls. STUDY DESIGN: A prospective, longitudinal single cohort study of patients treated with segmental instrumentation for fractures of the spine. Minimum 2-year follow-up. PATIENT SAMPLE: Seventy-five consecutive patients with unstable fractures of the thoracic, thoracolumbar and lumbar vertebrae, admitted to a level 1 trauma center. All patients sustained high-energy injuries: fifty-five (79%) were injured in motor vehicle accidents, 27 (38%) sustained two or more major additional injuries and 39 (56%) had neurological injuries. OUTCOME MEASURES: Perioperative morbidity and mortality, blood loss, surgical time; postoperative recovery, neurological recovery, complications, thromboembolic and pulmonary disease; long-term outcome measures of fusion, sagittal spinal alignment, construct survival, patient pain and function measures, and return to work and activity. METHODS: A longitudinal, prospective study of surgical outcome after segmental spinal instrumentation. Multifactorial assessment was carried out at prescribed intervals to a mean follow-up of 5 years (range, 2 to 8 years) from the time of surgery. Seventy patients were included in the final analysis. There were 17 thoracic, 36 thoracolumbar and 17 lumbar fractures. RESULTS: At 52 months mean follow-up, 57 of 62 patients (92%) had solid fusion with acceptable spinal alignment. Perioperative complications and mortality were less than expected, based on historical controls matched for injury severity. Rod and hook constructs had 97% good to excellent functional results, with no hardware complications. Six of 11 (55%) patients with short-segment pedicle instrumentation (SSPI) with no anterior column reconstruction had greater than 10 degrees of sagittal collapse during the fracture healing period. Twenty six of 36 neurologically injured patients (72%) experienced (mean) 1.5 Frankel grades recovery after decompression and stabilization. Residual neurological deficit determined return to work: 43 patients (70%) returned to work, 33 without restrictions, 10 with limitations. Five other patients (8%) were fit but unemployed. Fifteen percent experienced some form of hardware failure, but only three (5%) required revision. Hardware complications and fair to poor outcomes occurred after pedicle instrumentation without anterior reconstruction. Patients with anterior reconstruction had 100% construct survival, no sagittal deformity, and less pain. CONCLUSION: Segmental instrumentation allowed immediate mobilization of these severely injured patients, eliminating thromboembolic and pulmonary complications, and reducing overall morbidity and mortality. Segmental instrumentation produced a high rate of fusion with no rod breakage or hook failure. Pedicle screw constructs had a high rate of screw complications associated with anterior column insufficiency, but revision was not always necessary. Eighty percent of these severely injured patients were capable of returning to full-time employment, and 70% did so.

Endoscopic approaches to metastatic thoracic disease.

Traditional approaches to thoracic metastases and spinal cord compression have been well worked out and validated in the literature. Anterior decompression is clearly superior to laminectomy; vertebrectomy and reconstruction are indicated for sagittal collapse, instability, and pain; and surgical decompression is necessary in cases of bony impingement. The role of endoscopic and minimally invasive techniques in treatment of metastatic disease is evolving. Dr. Lieberman advocates the use of thoracoscopic anterior approaches as the principal application in these patients, whereas Dr. McLain has found that endoscopic assistance has vastly improved his results with posterolateral decompression. The two authors weigh the relative advantages and disadvantages of these approaches for the selected patient with metastatic thoracic disease.

Urgent surgical stabilization of spinal fractures in polytrauma patients.

STUDY DESIGN: A prospective, longitudinal study of multiply injured patients treated with segmental instrumentation for spinal fractures with a minimum 2-year follow-up. OBJECTIVES: To determine whether urgent stabilization of spinal fractures in severely injured patients increases the risk of surgery compared with early treatment and historical results. SUMMARY AND BACKGROUND DATA: Opinion in clinical studies is divided about whether operative treatment offers an advantage over nonoperative treatment in isolated spine fractures. Concomitant trauma is rarely discussed relative to decision making or surgical timing. Urgent stabilization of long-bone fractures improves survival and outcome in polytrauma patients. To date, urgent treatment of spine fractures in polytrauma patients has not been considered in the literature. METHODS: Seventy-five consecutive patients treated with segmental instrumentation for spinal trauma were observed prospectively to assess perioperative and longterm outcome. Twenty-seven patients with severe polytrauma (injury severity score, > 26) were separately analyzed. Perioperative and postoperative results were analyzed relative to timing of surgery, injury severity score, and surgical approach. Urgent treatment was defined as that provided within 24 hours of the spinal injury, and early treatment was defined as that provided between 24 and 72 hours after injury. RESULTS: Twenty-five patients (93%) sustained two or more major injuries in addition to the spine fracture, and 17 of 27 (63%) had neurologic injury. The mean injury severity score approached or exceeded the LD50 (50% expected mortality) in each group--36.0 for the early-treatment group and 42.0 for the urgent group--but only one patient in each group died. There were no deep venous thromboses, pulmonary emboli, neurologic injuries, decubiti, deep wound infections, or episodes of sepsis in either group. Blood loss for anterior procedures was significantly higher in the urgent group, but estimated blood loss for posterior procedures was similar for both groups. At 49 months' mean follow-up, no revisions were necessitated by the urgent spinal treatment. CONCLUSIONS: Urgent spinal stabilization is safe and appropriate in polytrauma patients when progressive neurologic deficit, thoracoabdominal trauma, or fracture instability increase the risks of delayed treatment.

Characteristics of pedicle screw loading. Effect of sagittal insertion angle on intrapedicular bending moments.

STUDY DESIGN: A bending analysis of pedicle screws inserted into vertebral body analogues. Intravertebral and intrapedicular pedicle screw bending moments were studied as a function of sagittal insertion angle. OBJECTIVES: To determine how the pedicle screw bending moment is affected by changes in the insertion angle. SUMMARY OF BACKGROUND DATA: There is a significant incidence of failure when pedicle screws are used to instrument unstable spinal segments. Extrinsic factors that affect screw bending failure have been poorly characterized. Previous work has demonstrated that intrapedicular pedicle screw bending moments are significantly affected by the sagittal location and depth of pedicle screw placement. METHODS: Pedicle screw transducers were inserted in analogue vertebrae at one of three orientations: 7 degrees cephalad (toward the superior endplate), 7 degrees caudal (toward the inferior endplate), or parallel to the superior endplate (control). An axial load was applied to the superior endplate of the vertebra, and screw bending moments were recorded directly from the transducers. RESULTS: Screws angled 7 degrees cephalad developed significantly greater mean intrapedicular bending moments compared with screws inserted caudal or control screws. There was no significant difference in bending moments realized within the vertebral body for the three screw positions. CONCLUSIONS: Angulating pedicle screws toward the superior endplate increased bending moments within the pedicle. If attention to optimal screw insertion technique can reduce bending moments and potential for screw failure without increasing morbidity, surgical risk, or operative time, then proper insertion technique takes on new importance.

Measurement of strain distributions within vertebral body sections by texture correlation.

STUDY DESIGN: A high-resolution strain measurement technique was applied to axially loaded parasagittal sections from thoracic spinal segments. OBJECTIVES: To establish a new experimental technique, develop data analysis procedures, characterize intrasample shear strain distributions, and measure intersample variability within a group of morphologically diverse samples. SUMMARY OF BACKGROUND DATA: Compression of intact vertebral bodies yields structural stiffness and strength, but not strain patterns within the trabecular bone. Finite element models yield trabecular strains but require uncertain boundary conditions and material properties. METHODS: Six spinal segments (T8-T10) were sliced in parasagittal sections 6-mm thick. Axial compression was applied in 25-N increments up to sample failure, then the load was removed. Contact radiographs of the samples were made at each loading level. Strain distributions within the central vertebral body were measured from the contact radiographs by an image correlation procedure. RESULTS: Intrasample shear strain probability distributions were log-normal at all load levels. Shear strains were concentrated directly inferior to the superior end-plate and adjacent to the anterior cortex, in regions where fractures are commonly seen clinically. Load removal restored overall sample shape, but measurable residual strains remained. CONCLUSIONS: This experimental model is a suitable means of studying low-energy vertebral fractures. The methods of data interpretation are consistent and reliable, and strain patterns correlate with clinical fracture patterns. Quantification of intersample variability provides guidelines for the design of future experiments, and the strain patterns form a basis for validation of finite element models. The results imply that strain uniformity is an important criterion in assessing risk of vertebral failure.

Characteristics of pedicle screw loading. Effect of surgical technique on intravertebral and intrapedicular bending moments.

STUDY DESIGN: A static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw load was studied as a function of variables in insertion technique. OBJECTIVES: To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagittal placement. BACKGROUND DATA: An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affect in situ loads incurred by pedicle screws. METHODS: Synthetic vertebral analogues were fabricated. Pedicle screws internally instrumented with strain gauges were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture. RESULTS: Screw bending moments within the pedicle increased 33% and 52% when screws were left 3 mm and 5 mm short of full insertion. Intrapedicular moments increased 20% to 29% in screws inserted superiorly or inferiorly within the pedicle. Thirty-five-millimeter screws developed intrapedicular moments 16% higher than 40-mm and 45-mm screws. CONCLUSIONS: In situ pedicle screw loads increased significantly as a direct result of variations in surgical technique. Screws left short of full insertion, placed off center in the sagittal plane of the pedicle, or less than 40 mm long developed increased intrapedicular bending moments.

The effect of boundary conditions on experimentally measured trabecular strain in the thoracic spine.

Vertebral bodies are the primary structural entities of the spine, and trabecular bone is the dominant material from which vertebral bodies are composed. Understanding the mechanical characteristics of vertebral trabecular bone, therefore, is of critical importance in the many clinical conditions that affect the spine. Numerous studies have loaded vertebral bodies to investigate the influence of trabecular bone characteristics on deformation and failure patterns, but the methods of load application have been inconsistent. These differences in the method of load application are a potential confounding factor in the interpretation of the experimental results. We investigated this problem by measuring the distribution of minimum principal strain and maximum shear strain magnitude within 6.35 mm thick samples cut from thoracic spine segments (T8-T10) and loaded to simulate three common experimental configurations. Measurements were made using the texture correlation technique, which extracts deformation patterns from digitized contact radiographs of samples under load. The three loading configurations examined were a three-body construct, a single vertebral body loaded through sectioned intervertebral discs, and polymethylmethacrylate molded directly to the endplates. Results indicate that from both probability and spatial distribution standpoints the best simulation of in vivo loading generates the least uniform strains. Loading through disc remnants or through plastic molded to the endplates causes increasing degrees of strain homogenization. This result has implications not only for the design of experiments involving spinal loading, but also for theories concerning the adaptation of trabecular bone to functional loads.

Revision of failed pedicle screws using hydroxyapatite cement. A biomechanical analysis.

STUDY DESIGN: The biomechanical influence of in situ setting hydroxyapatite cement was examined for use in pedicle screw revision surgery. Pull-out testing of control and pedicle screws augmented with hydroxyapatite cement was performed in human cadaver vertebrae. OBJECTIVES: To determine the immediate effect of using hydroxyapatite cement to augment revision pedicle screws after failure of the primary pedicle screw fixation. SUMMARY OF BACKGROUND DATA: The potential problems associated with using polymethylmethacrylate to augment revision pedicular instrumentation have prompted the search for other solutions. The introduction of resorbable hydroxyapatite pastes may have provided new biocompatible solutions for pedicle screw revision. METHODS: Ten human cadaver vertebrae were instrumented with 6.0-mm pedicle screws in each pedicle. The screws were loaded to failure in axial tension (pull-out). The failed pedicles then were instrumented with 7.0-mm pedicle screws, either augmented with hydroxyapatite cement or nonaugmented, which also were loaded to failure. Finally, the nonaugmented 7.0-mm screw hole was reinstrumented with a hydroxyapatite cement-augmented, 7.0-mm pedicle screw and loaded to failure. RESULTS: The pull-out strength of the 7.0-mm, hydroxyapatite cement-augmented screws was 325% (P = 2.9 x 10(-5)) of that of the 6.0-mm control screws, whereas the strength of the 7.0-mm nonaugmented screws was only 73% (P = 2.0 x 10(-2)) of that of the 6.0-mm control screws. The 7.0-mm screws augmented with hydroxyapatite cement also were able to salvage 7.0-mm pull-out sites to 384% (P = 6.9E-5) of the pull-out strength of the 7.0-mm nonaugmented screws. CONCLUSIONS: Hydroxyapatite cement may be a mechanically viable alternative to polymethyl methacrylate for augmenting revision pedicular instrumentation and should be considered for future experimental, animal, and clinical testing.

Newer management options in patients with spinal metastasis.

For some patients with spinal metastasis and spinal cord compression, newer surgical techniques are better than laminectomy or radiotherapy alone in relieving pain and restoring function. While radiotherapy remains the standard for spinal metastases due to myeloma, lymphoma, and many types of adenocarcinoma, proper surgical treatment can significantly improve function and outcome in selected patients.

Endoscopically assisted decompression for metastatic thoracic neoplasms.

STUDY DESIGN: The author describes a technique for complete vertebrectomy and anterior decompression followed by a formal anterior column reconstruction, using readily available endoscopic instruments. This procedure is indicated in patients with radioresistant metastasis of the thoracic spine, particularly those involving the upper thoracic segments where a thoracotomy is difficult and causes a high rate of morbidity. This is also a suitable technique for patients with pulmonary disease who cannot tolerate a standard thoracotomy. OBJECTIVES: To demonstrate the feasibility and potential benefits of endoscopically controlled decompression through an extrapleural, posterolateral approach. SUMMARY OF BACKGROUND DATA: Posterolateral decompression of the thoracic spine offers potential advantages in comparison with traditional anterior-posterior procedures combining thoracotomy and posterior instrumentation, including decreased operative time, decreased morbidity, and reduced hospital stay. Results of previous studies have not demonstrated the same benefit for posterolateral decompression as for anterior vertebrectomy and decompression. Drawbacks to the traditional posterolateral decompressions have included poor visualization of the spinal cord and anterior tumor, poor access to tumor on the side contralateral to the approach, and the need to manipulate the spinal cord to completely remove adjacent tumor and tumor adherent to the dura. METHODS: Surgical indications, rationale, and technique are provided, and initial clinical results are described. RESULTS: Transpedicular decompression using endoscopy is described in five patients. The mean operative time for the combined procedure was 7.25 hours, with a mean blood loss of 1800 mL. Neurologic recovery and maintenance were excellent. Inpatient days averaged 7.5, and intensive care days averaged 2. One patient died of disease 8 months after surgery, and four were living, with disease, 3-24 months after surgery. CONCLUSIONS: Endoscopically assisted decompression can reduce morbidity, hospital stay, and treatment costs while matching the efficacy of traditional combined procedures. Endoscopy provides a readily available and easily applied tool that dramatically improves the surgeon's vision, providing light, magnification, and a direct view of remote structures.

Variation in surgical opinion regarding management of selected cervical spine injuries. A preliminary study.

STUDY DESIGN: The opinions of orthopedic surgeons and neurosurgeons were compared regarding appropriate management of selected cervical injuries and the timing of stabilization. OBJECTIVE: To determine whether there is consistency of opinion regarding the management of cervical trauma. SUMMARY OF BACKGROUND DATA: Numerous forms of management for cervical trauma exist, but there are few consistent recommendations. No previous study has been done to determine uniformity of preferences of the surgeons who manage these injuries. METHODS: Thirty-one orthopedic surgeons and neurosurgeons were given a brief clinical situation and pertinent radiographic studies of five selected cervical injuries. Management options included halo and nonhalo orthoses, traction, and various forms of anterior and/or posterior procedures. The surgeons rated, in whole numbers from 1 to 10, their opinions on the appropriateness of each technique. Each surgeon was given a case of a "generic" cervical injury, in which stabilization was required and for which preoperative alignment was adequate in traction. They gave opinions on the timing of stabilization, with a choice of four time frames. Four neurologic situations were rated, ranging from intact to complete cord injury. RESULTS: Of 46 possible responses to the five test cases regarding appropriateness, 18 ranged from 1 to 10, the largest possible variation. Only 2 had a range of 5 or less, implying better consensus among tested surgeons. Mean values ranged from 1.9 to 9.5. Agreementamong respondents regarding appropriateness was slight with a range of kappa statistics from 0.09 to 0.14. Of 16 possible responses regarding timing, 14 were within a range of 8 or higher. Within 24 to 72 hours was the generally preferred time frame, with all possible responses showing a range of 3 or 4. Results of a multiple analysis of variance showed no significant differences among respondents. CONCLUSION: There is a large variety of opinion regarding appropriateness of specific operative and nonoperative management procedures and surgical timing among the surgeons polled who manage cervical trauma. This implies that there is no widely accepted standard management procedure for many of these injuries.

Reinforcement of thoracolumbar burst fractures with calcium phosphate cement. A biomechanical study.

STUDY DESIGN: A biomechanical study on the stabilization of thoracolumbar burst fractures. OBJECTIVE: To demonstrate that the addition of a calcium phosphate cement into the fractured vertebral body through a transpedicular approach is a feasible technique that improves the stiffness of a transpedicular screw construct. SUMMARY OF BACKGROUND DATA: Short segment pedicle screw instrumentation is a commonly used method for reduction and stabilization of unstable burst fractures. Recent investigators, however, have reported a high rate of instrumentation failure and sagittal collapse when there is a loss of anterior column support. In this study, the ability of a new hydroxyapatite cement to augment anterior column support was investigated in a burst fracture model. METHODS: A cadaveric L1 burst fracture model was stabilized using short segment pedicle screw instrumentation. Specially instrumented-pedicle screws recorded screw-bending moments. The L1 vertebral body was reinforced with the hydroxyapatite cement through a transpedicular approach. Mechanical testing of the instrumented and instrumented-reinforced constructs were performed in flexion, extension, side bending, and torsion. Construct stiffness and screw-bending moments were recorded. RESULTS: Transpedicular vertebral body reconstruction with hydroxyapatite cement reduced pedicle screw-bending moments by 59% in flexion and 38% in extension. Mean initial stiffness in the flexion-extension plane was increased by 40% (P < 0.05). There were no statistically significant differences in these parameters with lateral bending or torsional movements. CONCLUSIONS: This hydroxyapatite cement compound augments anterior column stability in a burst fracture model. This technique may improve outcomes in burst fracture patients without the need for a secondary anterior approach.

Missed cervical dissociation--recognizing and avoiding potential disaster.

Complete cervical disruptions are high-energy injuries often associated with polytrauma and spinal cord injury. Because these injuries disrupt all anterior and posterior stabilizers, they result in a highly unstable spine, and the injuries are usually apparent on screening radiographs. Patients with these injuries must be identified and protected during the multiple diagnostic and surgical procedures they may require during their initial evaluation and treatment. Emergency procedures must be carefully prioritized relative to other life-threatening injuries; formal evaluation of the cervical spine may be carried out before, after, or in stages around other urgently indicated procedures. Until the cervical spine is cleared, careful observation of precautions can avoid disasterous complications in even the most unstable situation. A case of complete cervical disruption in a neurologically intact, hemodynamically unstable patient is presented for discussion. For polytraumatized patients with cervical dissociation, combined anterior and posterior stabilization is the treatment of choice.

Mechanoreceptor endings in human thoracic and lumbar facet joints.

STUDY DESIGN: Histologic analysis of normal human facet capsules to determine the density and distribution of encapsulated nerve endings in the thoracic and lumbar spine. OBJECTIVES: To quantify the extent of mechanoreceptor innervation in normal facet tissues and determine the relative distribution of three specific receptor types with respect to thoracic and lumbar segments. SUMMARY OF BACKGROUND DATA: Ongoing studies of spinal innervation have shown that human facet tissues contain mechanoreceptive endings capable of detecting motion and tissue distortion. The hypothesis has been advanced that spinal proprioception may play a role in modulating protective muscular reflexes that prevent injury or facilitate healing. METHODS: Whole facet capsules harvested from seven healthy adult patients were processed using a gold chloride staining method and cut into 35-micron sections for histologic analysis. No sampling was performed; all sections were analyzed. Receptor endings were classified by the method of Freeman and Wyke if they met the following three criteria: 1) encapsulation, 2) identifiable morphometry, and 3) consistent morphometry on serial sections. RESULTS: One Type 1 and four Type 2 endings were identified among 10 thoracic facet capsules. Five Type 1, six Type 2, and one Type 3 ending were identified among 13 lumbar facet capsules. Occasional atypical receptive endings were noted that did not fit the established classification. Unencapsulated free nerve endings were seen in every specimen, but were not quantified. CONCLUSIONS: Encapsulated nerve endings are believed to be primarily mechanosensitive and may provide proprioceptive and protective information to the central nervous system regarding joint function and position. A consistent, but small population of receptors has been found previously in cervical facets, but innervation of the thoracic and lumbar levels is less consistent. This suggests that proprioceptive function in the thoracic and lumbar spine is less refined and, perhaps, less critical than in the cervical spine.