Robert F. McLain

Mechanoreceptor endings in human cervical facet joints

Twenty-one cervical facet capsules, taken from three normal human subjects, were examined to determine the type, density, and distribution of mechanoreceptive nerve endings in these tissues. Clearly identifiable mechanoreceptors were found in 17 of 21 specimens and were classified according to the scheme for encapsulated nerve endings established by Freeman and Wyke. Eleven Type I, 20 Type II, and 5 Type III receptors were identified, as well as a number of small, unencapsulated nerve endings. Type I receptors were small globular structures measuring 25-50 microns in diameter. Type II receptors varied in size and contour, but were characterized by their oblong shape and broad, lamellated capsule. Type III receptors were relatively large oblong structures with an amorphous capsule, within which a reticular meshwork of fine neurites was embedded. Free (nociceptive) nerve endings were found in subsynovial loose areolar and dense capsular tissues. The presence of mechanoreceptive and nociceptive nerve endings in cervical facet capsules proves that these tissues are monitored by the central nervous system and implies that neural input from the facets is important to proprioception and pain sensation in the cervical spine. Previous studies have suggested that protection muscular reflexes modulated by these types of mechanoreceptors are important in preventing joint instability and degeneration. It is suggested that the surgeon take steps to avoid inadvertently damaging these tissues when exposing the cervical spine.

Primary pedicle screw augmentation in osteoporotic lumbar vertebrae: biomechanical analysis of pedicle fixation strength.

Study Design. Pedicle screw pullout testing in osteoporotic and control human cadaveric vertebrae, comparing augmented and control vertebrae. Objective. To compare the pullout strengths of pedicle screws fixed in osteoporotic vertebrae using polymethyl methacrylate delivered by 2 augmentation techniques, a standard transpedicular approach and kyphoplasty type approach. Summary of Background Data. Pedicle screw instrumentation of the osteoporotic spine carries an increased risk of screw loosening, pullout, and fixation failure. Osteoporosis is often cited as a contraindication for pedicle screw fixation. Augmentation of the vertebral pedicle and body using polymethyl methacrylate may improve fixation strength and construct survival in the osteoporotic vertebrae. While the utility of polymethyl methacrylate has been demonstrated for salvage of screws that have been pulled out, the effect of the cement technique on pullout strength in osteoporotic vertebrae has not been previously studied. Methods. Thirteen osteoporotic and 9 healthy human lumbar vertebrae were tested. All specimens were instrumented with pedicle screws using a uniform technique. Osteoporotic pedicles were augmented with polymethyl methacrylate using either a kyphoplasty type technique or a transpedicular augmentation technique. Screws were tested in a paired testing array, randomly assigning the augmentation techniques to opposite sides of each vertebra. Pullout to failure was performed either primarily or after a 5000-cycle tangential fatigue conditioning exposure. After testing, following screw removal, specimens were cut in the axial plane through the center of the vertebral body to inspect the cement distribution. Results. Pedicle screws placed in osteoporotic vertebrae had higher pullout loads when augmented with the kyphoplasty technique compared to transpedicular augmentation (1414 ± 338 versus 756 ± 300 N, respectively; P < 0.001). An unpaired t test showed that fatigued pedicle screws in osteoporotic vertebrae augmented by kyphoplasty showed higher pullout resistance than those placed in healthy control vertebrae (P = 0.002). Both kyphoplasty type augmentation (P = 0.007) and transpedicular augmentation (P = 0.02) increased pullout loads compared to pedicle screws placed in nonaugmented osteoporotic vertebrae when tested after fatigue cycling. Conclusions. Pedicle screw augmentation with polymethyl methacrylate improves the initial fixation strength and fatigue strength of instrumentation in osteoporotic vertebrae. Pedicle screws augmented using the kyphoplasty technique had significantly greater pullout strength than those augmented with transpedicular augmentation technique and those placed in healthy control vertebrae with no augmentation.

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.

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.

Variation in Surgical Decision Making For Degenerative Spinal Disorders. Part Ii: Cervical Spine

Study Design. Survey-based descriptive study. Objective. To study relationships between surgeon-specific factors and surgical approach to degenerative diseases of the cervical spine. Summary of Background Data. Geographic variations in the rates of cervical spine surgery are significant within the United States. Although surgeon density correlates with the rates of spinal surgery, other reasons for variation such as surgeon-specific factors are poorly understood. Methods. A total of 22 orthopedic surgeons and 8 neurosurgeons of varied ages and geographic regions answered questions regarding the need for surgery, surgical approach, and use of fusion and instrumentation for 5 simulated cases. Cases included: (1) single-level disc herniation with osteophyte and radiculopathy, (2) single-level pseudarthrosis with axial neck pain, (3) multilevel stenosis with radiculopathy and neutral lordosis, (4) multilevel stenosis with myelopathy and neutral lordosis, and (5) multilevel stenosis with myelopathy and marked kyphosis. The effects of surgeon age and training background on surgical decision making were analyzed using an independent samples t test and Fisher exact test, respectively. Results. The greatest agreement occurred for the single-level disc herniation, with all surgeons choosing an anterior discectomy, and 28 of the 29 respondents recommending fusion. Younger surgeons recommended instrumentation more often for all cases, reaching significance for the case of multilevel stenosis with myelopathy and neutral lordosis (Fisher exact test P = 0.02). Differences in recommendation for fusion, instrumentation, and the use of a posterior approach between orthopedic and neurosurgeons were limited. Conclusions. Variations in surgical procedures for cervical degenerative disease may depend on the clinical condition. Although this study found strong agreement in treatment approach to single-level disc herniation, significant variation was seen for the other degenerative conditions of the cervical spine. While differences in recommendation for fusion were not clearly associated with surgeon age, there was a trend toward the higher use of instrumentation by younger surgeons. Previously documented geographic variation may result in part from a lack of consensus regarding appropriate treatment techniques for certain degenerative conditions of the cervical spine, as well as surgeon-specific factors.

Short-segment pedicle instrumentation. Biomechanical analysis of supplemental hook fixation

Study Design This biomechanical study of fractures in cadaver vertebrae used specially designed pedicle screws to determine screw strains during loading of two different fixation constructs. Objectives The authors determined the relative benefit of adding offset sublaminar hooks to standard pedicle screw constructs to reduce screw bending moments and prevent fixation failure and sagittal collapse. Summary of Background Data Clinical studies have demonstrated a high incidence of early screw failure in short-segment pedicle instrumentation constructs used to treat unstable burst fractures. Strategies to prevent early construct failure include longer constructs, anterior strut graft reconstruction, and use of offset sublaminar hooks at the ends of standard short-segment pedicle instrumentation constructs. Methods Human cadaver spines with an L1 burst fracture were instrumented with a standard short-segment pedicle instrumentation construct using specially instrumented pedicle screws. Mechanical testing was carried out in flexion, extension, side bending, and torsion, and stiffness and screw bending moments were recorded. Offset hooks were applied initially, then removed and testing repeated. Stiffness data were compared to intact and postfracture results, and between augmented and standard constructs. Results Addition of offset laminar hooks, supralaminar at T11 and infralaminar at L2, to standard short-segment pedicle instrumentation constructs increased stiffness in flexion by 268%, in extension by 223%, in side bending by 161%, and in torsion by 155% (all were significant except torsion). Sublaminar hooks also reduced pedicle screw bending moments to roughly 50% of standard in both flexion and extension (P < 0.05). Conclusions Supplemental offset hooks significantly increase construct stiffness without sacrificing principles of short-segment pedicle instrumentation, and absorb some part of the construct strain, thereby reducing pedicle screw bending moments and the likelihood of postyield deformation and clinical failure.

Primary Tumors of the Spine.

Eighty-two cases of primary neoplasms of the spine, diagnosed and treated at the University of Iowa, were reviewed in an attempt to identify features of diagnostic and prognostic importance, and to evaluate the effectiveness of surgical treatment with respect to survival. Thirty-one benign and 51 malignant tumors were identified. The mean follow-up in benign lesions was 9.7 years and 3.8 years in malignant lesions. Plain roentgenograms demonstrated the spinal lesion in 81 of 82 cases (99%). All spinal segments were involved, the cervical spine least frequently. Malignancy proved to be associated with an older age at diagnosis, a higher incidence of neurologic deficit, and a higher incidence of occurrence in the vertebral body. Five-year survival for patients with benign tumors was 86%, with no significant relationship between type of surgery and survival. Five-year survival in malignant lesions did correlate with the extent of initial surgery and with the tumor type. Five-year survival in patients undergoing curettage for malignancy was nil, in those undergoing incomplete resection, 18.7%, and in patients having complete excision, 75%. Plain anteroposterior and lateral roentgenograms should be obtained as a screening study in patients with persistent or atypical back pain or neurologic signs. CT scanning, myelography, and magnetic resonance imaging studies may be utilized to establish the physical margins of the lesion and to evaluate cord impingement. Surgical extirpation should be attempted whenever possible in malignant and benign aggressive lesions. In this series, the prolonged survival seen with complete excision justifies an aggressive surgical approach to the treatment of these tumors.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative morphometry of L4 vertebrae: comparison of large animal models for the human lumbar spine.

Study Design. Anatomic analysis of L4 vertebral morphometry comparing specimens harvested from humans and five common large animal species. Objective. To compare fundamental structural similarities and differences in the vertebral bodies of commonly used experimental animals relative to human vertebrae. Summary of Background Data. Animal models are commonly used for assessment of spine fusion, instrumentation techniques, and vertebral bone biology. Among the animals used, the lumbar vertebrae exhibit considerable anatomic variability. The goal of this study was to determine which of the animals commonly used for spine research is best suited as an anatomic model for the human lumbar spine. Methods. Morphometric features of the L4 vertebrae of five common research animals were compared with those of the human L4 vertebrae. Mature canines, immature pigs, mature micropigs, mature dairy goats, and mature sheep were analyzed. These species were chosen because they are commonly selected research animals, and most research facilities do not need to be modified to use them. The samples included ten L4 vertebrae of each animal species and seven human L4 vertebrae. Each specimen was meticulously cleaned of all soft tissue. The measurements were grouped into vertebral body parameters, neural canal dimensions, and pedicle and facet morphometery. The mean of each anatomic measurement was compared using a single factor analysis of variance and a Scheffe’s post hoc test, with 0.05 denoting significance. Results. The human vertebral body was significantly wider and deeper in the anteroposterior plane than any of the animals studied. However, the mean vertebral body height of the sheep and goat significantly exceeded that of the human specimens. The mean pedicle angle of every animal species was significantly greater than that of the human. The mean pedicle width of the micropig and goat were significantly narrower than the human pedicles, and the dog specimens lacked a definable pedicle altogether. There was no significant difference in mean pedicle width between any of the remaining species and the human specimens. Facet tropism and radius of curvature of the sheep and goat specimens differed significantly from the remaining selections. Conclusions. When posterior pedicle instrumentation is part of a testing protocol, the increased pedicle angle and lack of vertebral body depth found in all animals studied must be kept in mind. In addition, when testing interbody cages designed to stabilize the spine and promote fusion, one must be aware of the decreased vertebral body depth and width in these animals, as compared with humans. Physeal defects in the immature pig may alter specific biomechanical results during failure or fatigue testing, or in basic studies of vertebral bone material properties. In all cases, instrumentation and hardware must be sized appropriately to the selected model to provide meaningful results.

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.

Functional outcomes after surgery for spinal fractures: return to work and activity.

Object of Study. The literature regarding surgical treatment’s impact on patient function after spinal fracture is sparse. Some authors have speculated that operative injury—the dissection of paraspinous muscle tissue, damage to spinal motion segments, implantation of spinal devices—may impair functional recovery in spine trauma patients. Nonoperative care has produced satisfactory results in some hands, but results are difficult to reproduce, treatment is resource-intensive, and functional outcomes are poorly documented. This study reports return to work and functional recovery in a 5-year follow-up of severely injured patients treated with segmental spinal instrumentation. Materials and Methods. Seventy consecutive patients treated with Cotrel Dubousset instrumentation for unstable thoracic, thoracolumbar, and lumbar spine fractures were followed-up. All had high-energy trauma and were admitted directly to a level 1 university trauma center; 38% were polytraumatized; and 56% had neurologic injuries. Indications for surgery included: (1) segmental instability; (2) incomplete or progressive neurologic injuries with residual spinal canal compromise; (3) concomitant injuries precluding cast treatment; and (4) polytrauma. Two patients died and six were lost to follow-up, leaving 62 (91%) for assessment at a mean 5-year follow-up (range 2–8 y). Clinical outcome has been reported. Functional recovery was assessed based on return to work, level of work, and level of daily activity. Results. Despite the severity of spinal and concomitant injuries, 70% of patients returned to full-time work and another 8% were considered capable: 54% to their previous level of employment without restrictions and 16% to full-time, but lighter, jobs. Twenty-two percent were working part-time or not at all, and 8% were unemployed despite unrestricted functional status. Work status correlated directly with neurologic impairment (P < 0.00005) and was not related to level of injury, hardware failure, extent of surgical dissection, or construct pattern. Of patients with limitations, 18% were limited by pain and 27% by neurologic injury. Conclusion. Neurologic injury had a greater impact on functional outcome than any other variable. Patients limited by pain were more often impaired by residual radicular and neuropathic symptoms than by back pain. Impairment was not related to the extent of either the surgical incision or the instrumentation. Patients with persistent back pain generally had an identifiable and correctable mechanical problem—sagittal imbalance, pseudarthrosis, or persistent instability—as the underlying cause. Our series of trauma patients was predominantly young and male. Among this cohort, individual characteristics of occupation (often physical laborers and craftsmen) and judgment (criminal convictions and incarceration) may have restricted opportunities for re-employment in 40% of the entire study group.