Morphology and kinematics of the baboon upper cervical spine. A model of the atlantoaxial complex

Osteology of the Hamadryas Baboon (Papio hamadryas)

Besides living as a free-ranging primate in the horn of Africa and the Arabian Peninsula, the hamadryas baboon has an important place in zoos and can be found in biomedical research centers worldwide. To be valuable as a non-human primate laboratory model for man, its anatomy should be portrayed in detail, allowing for the correct interpretation and translation of obtained research results. Reviewing the literature on the use of the baboon in biomedical research revealed that very limited anatomical works on this species are available. Anatomical atlases are incomplete, use archaic nomenclature and fail to provide high-definition color photographs. Therefore, the skeletons of two male hamadryas baboons were prepared by manually removing as much soft tissues as possible followed by maceration in warm water to which enzyme-containing washing powder was added. The bones were bleached with hydrogen peroxide and degreased by means of methylene chloride. Photographs of the various bones were taken, and the anatomical structures were identified using the latest version of the Nomina Anatomica Veterinaria. As such, the present article shows 31 annotated multipanel figures. The skeleton of the hamadryas baboon generally parallels the human skeleton, but some remarkable differences have been noticed. If these are taken into consideration when evaluating the results of experiments using the hamadryas baboon, justified conclusions can be drawn.

Biomechanical in vitro evaluation of the kangaroo spine in comparison with human spinal data

On the basis of the kangaroo's pseudo-biped locomotion and its upright position, it could be assumed that the kangaroo might be an interesting model for spine research and that it may serve as a reasonable surrogate model for biomechanical in vitro tests. The purpose of this in vitro study was to provide biomechanical properties of the kangaroo spine and compare them with human spinal data from the literature. In addition, references to already published kangaroo anatomical spinal parameters will be discussed. Thirteen kangaroo spines from C4 to S4 were sectioned into single-motion segments. The specimens were tested by a spine tester under pure moments. The range of motion and neutral zone of each segment were determined in flexion and extension, right and left lateral bending and left and right axial rotation. Overall, we found greater flexibility in the kangaroo spine compared to the human spine. Similarities were only found in the cervical, lower thoracic and lumbar spinal regions. The range of motion of the kangaroo and human spines displayed comparable trends in the cervical (C4-C7), lower thoracic and lumbar regions independent of the motion plane. In the upper and middle thoracic regions, the flexibility of the kangaroo spine was considerably larger. These results suggested that the kangaroo specimens could be considered to be a surrogate, but only in particular cases, for biomechanical in vitro tests.

C1 fracture: Analysis of consolidation and complications rates in a prospective multicenter series

INTRODUCTION

Three types of C1 fracture have been described, according to location: type 1 (anterior or posterior arc), type 2 (Jefferson: anterior and posterior arc), and type 3 (lateral mass). Stability depends on transverse ligament integrity. The main aim of the present study was to analyze complications and consolidation rates according to fracture type, age and treatment.

MATERIAL AND METHODS

The French Society of Spinal Surgery (SFCR) performed a multicenter prospective study on C1-C2 trauma. All patients with recent fracture diagnosed on CT were included. Consolidation on CT was studied at 3 months and 1 year. Medical, neurologic, infectious and mechanical complications were inventoried using the KEOPS data-base.

RESULTS

Sixty-three of the 417 patients (15.1%) had C1 fracture: type 1 (33.3%), type 2 (38.1%), or type 3 (28.6%). The transverse ligament was intact in 53.9% of cases. Treatment was non-operative in 63.5% of cases, surgical in 27.0%, and surgical after failure of non-operative treatment in 9.5%. There were 8 medical complications, more frequently in patients aged >70 years, following surgery (p<0.0001). The consolidation rate was 84.2% with non-operative treatment, 100% for primary surgery, and 33.3% for secondary surgery (p=0.002). There were 10 cases of non-union, in 4.8% of type 1, 13.6% of type 2 and 33.3% of type 3 fractures (p=0.001).

CONCLUSION

Medical complications showed association with age and with type of treatment. Non-operative treatment was suited to types 1, 2 and 3 with minimal displacement and intact transverse ligament. C1-C2 fusion was suited to displaced unstable type 2 fracture. Displaced type 3 fracture incurred risk of non-union. Early surgery may be recommended.

LEVEL OF EVIDENCE

III.

Anatomy of large animal spines and its comparison to the human spine: a systematic review

Animal models have been commonly used for in vivo and in vitro spinal research. However, the extent to which animal models resemble the human spine has not been well known. We conducted a systematic review to compare the morphometric features of vertebrae between human and animal species, so as to give some suggestions on how to choose an appropriate animal model in spine research. A literature search of all English language peer-reviewed publications was conducted using PubMed, OVID, Springer and Elsevier (Science Direct) for the years 1980-2008. Two reviewers extracted data on the anatomy of large animal spines from the identified articles. Each anatomical study of animals had to include at least three vertebral levels. The anatomical data from all animal studies were compared with the existing data of the human spine in the literature. Of the papers retrieved, seven were included in the review. The animals in the studies involved baboon, sheep, porcine, calf and deer. Distinct anatomical differences of vertebrae were found between the human and each large animal spine. In cervical region, spines of the baboon and human are more similar as compared to other animals. In thoracic and lumbar regions, the mean pedicle height of all animals was greater than the human pedicles. There was similar mean pedicle width between animal and the human specimens, except in thoracic segments of sheep. The human spinal canal was wider and deeper in the anteroposterior plane than any of the animals. The mean human vertebral body width and depth were greater than that of the animals except in upper thoracic segments of the deer. However, the mean vertebral body height was lower than that of all animals. This paper provides a comprehensive review to compare vertebrae geometries of experimental animal models to the human vertebrae, and will help for choosing animal model in vivo and in vitro spine research. When the animal selected for spine research, the structural similarities and differences found in the animal studies must be kept in mind.

Thoracolumbar burst fracture with complete paraplegia: rationale for second-stage anterior decompression and fusion regarding functional outcome

BACKGROUND

Appropriate management of thoracolumbar injury with complete paraplegia remains controversial. Purpose of present study is to study whether advantages are worth the morbidity associated with staged anterior decompression in these patients.

MATERIALS AND METHODS

Forty patients (90% male) with fracture of T12 (32 cases) and L1 (8 cases) with complete paraplegia underwent transpedicular fixation. Average age of patients was 42 years (range 13-57 years). Most common fracture pattern was type A3.1 (55%). Rational staged anterior decompression was done in 20 cases. One group received transpedicular fixation (n = 20) and another fixation and staged decompression (n = 20). Average follow-up was 2.5 years.

RESULTS

Mean functional independence measurement (FIM) score was 98 in fixation group and 112 in decompression group; mean neurological recovery as measured by American Spinal Injury Association (ASIA) grade was 1.3 and 1.75, respectively. Incidence of postoperative complications was 20% and 60%, respectively. Sphincter control did not recover in either group.

CONCLUSIONS

Rehabilitation is better after staged anterior decompression and fusion in burst fracture of thoracolumbar junction with complete paraplegia.

Anatomy and biomechanics of normal craniovertebral junction (a) and biomechanics of stabilization (b)

INTRODUCTION

A knowledge of the bony configuration, ligamentous attachments, joint articulations, vascular supply, muscle function, and lymphatic drainage as well as the kinetic anatomy of the craniocervical junction is necessary to understand the etiology of abnormalities in this area and their treatment.

RESULTS AND DISCUSSION

The craniovertebral junction (CVJ) is the most mobile of the upper cervical spine especially in children. It is uniquely adapted for stability and motion. The bony anatomy and the normal biomechanics of the CVJ in children are presented and subsequently the biomechanics of complex stabilization. Our review of more than 600 children who required stabilization is presented.

Measurement of vertebral kinematics using noninvasive image matching method-validation and application

STUDY DESIGN

In vitro and in vivo laboratory study.

OBJECTIVE

To validate a dual fluoroscopic image matching technique for measurement of in vivo spine kinematics.

SUMMARY OF BACKGROUND DATA

Accurate knowledge of the spinal structural functions is critical to understand the biomechanical factors that affect spinal pathology. Many studies have investigated vertebral motion both in vitro and in vivo. However, determination of in vivo motion of the vertebrae under physiologic loading conditions remains a challenge in biomedical engineering because of the limitations of current technology and the complicated anatomy of the spine.

METHODS

In in vitro validation, an ovine spine was moved to a known distance in a known speed by an MTS machine. The dual fluoroscopic system was used to capture the spine motion and reproduce the moving distance and speed. In in vivo validation, a living subject moved the spine in various positions under weightbearing. The fluoroscopes were used to reproduce the in vivo spine positions 5 times. The standard deviations in translation and orientation of the 5 measurements were used to evaluate the repeatability of technique.

RESULTS

The translation positions of the ovine spine could be determined with a mean accuracy less than 0.40 mm for the image matching technique using magnetic resonance image-based vertebral models. The spine speed could be reproduced within an accuracy of 0.2 mm/s. The repeatability of the method in reproducing in vivo human spine 6DOF kinematics was less than 0.3 mm in translation and less than 0.7 degrees in orientation.

CONCLUSION

The image matching technique was accurate and repeatable for noninvasive measurement of spine vertebral motion. The technique could be a useful tool for determination of vertebral positions and orientations before and after surgical treatment of spinal pathology to evaluate and improve the efficacy of the various surgical methods in restoring normal spine function.

Neural space and biomechanical integrity of the developing cervical spine in compression

STUDY DESIGN

A factorial study design was used to examine the biomechanical and neuroprotective integrity of the cervical spine throughout maturation using a postmortem baboon model.

OBJECTIVE

To investigate changes with spinal development that affect the neuroprotective ability of the cervical spine in compressive loading.

SUMMARY OF BACKGROUND DATA

Child spinal cord injuries claim and debilitate thousands of children in the United States each year. Many of these injuries are diagnostically and mechanistically difficult to classify, treat, and prevent. Biomechanical studies on maturing spinal tissues have identified decreased stiffness and tolerance characteristics for children compared with adults. Unfortunately, while neurologic deficit typically dictates functional outcome, no previous studies have examined the neuroprotective role of the pediatric cervical spine.

METHODS

Twenty-two postmortem baboon cervical spines across the developmental age spectrum were tested. Two functional spinal unit segments (Oc-C2, C3-C5, and C6-T1) were instrumented with transducers to measure dynamic changes in the spinal canal. These tissues were compressed to 70% strain dynamically, and the resultant mechanics and spinal canal occlusions were recorded.

RESULTS

Classic injury patterns were observed in all of the specimens tested. The compressive mechanics exhibited a significant age relationship (P < 0.0001). Furthermore, while the peak-percent spinal canal occlusion was not age dependent, the percent occlusion just before failure did demonstrate a significant decrease with advancing age (P = 0.0001).

CONCLUSIONS

The neuroprotective ability of the cervical spine preceding failure appears to be age dependent, where the young spine can produce greater spinal canal occlusions without failure than its adult counterpart. The overall percent of the spinal canal occluded during a compression injury was not age dependent; however, these data reveal the neuroprotective ability of the child spine to be more sensitive as an injury predictor than the biomechanical fracture data.

Limitations of the cervical porcine spine in evaluating spinal implants in comparison with human cervical spinal segments: a biomechanical in vitro comparison of porcine and human cervical spine specimens with different instrumentation techniques

STUDY DESIGN

Porcine and human cervical spine specimens were in vitro biomechanically compared with different instrumentation techniques.

OBJECTIVES

To evaluate whether subaxial porcine cervical spines are a valid model for implant testing in a single level corpectomy.

SUMMARY OF BACKGROUND DATA

Biomechanical in vitro tests are widely used for implant tests, mainly with human spine specimens. The availability of human cadavers is limited and the properties of the specimen regarding age, bone mineral density, and grade of degenerative changes is inhomogeneous.

METHODS

Six porcine and six human cervical specimens were loaded nondestructively with pure moments: 1) in an intact state; 2) after a corpectomy of C5 and substitution by a cage with integrated force sensor; 3) after additional instrumentation with a posterior screw and rod system with: a) lateral mass and b) pedicle screws; 4) after instrumentation with an anterior plate; and 5) with a circumferential instrumentation. The unconstrained motion and the axial loads occurring in the corpectomy gap were measured, as well as the bone mineral density of the specimen before testing.

RESULTS

The range of motion in the intact state, as well as for the different instrumentations, was comparable for flexion-extension. In lateral bending and axial rotation, marked differences in the intact state as well as for pedicle screw instrumentations occurred.

CONCLUSIONS

The subaxial porcine cervical spine is a potential model in flexion-extension because of its biomechanical similarity. For lateral bending and axial rotation, the marked differences severly restrict the comparability.

Spinal maturation affects vertebral compressive mechanics and vBMD with sex dependence

The effects of natural aging on the mechanics of the spine are far better understood for the mature adult spine than for the developing (immature) spine. Throughout its chondrification and ossification, the vertebra, which is the primary structural unit of the spine, undergoes enormous cellular, biochemical, and structural changes that should strongly influence its biomechanical response to external forces. Unfortunately, very little data exist for the mechanics of immature vertebrae. Vertebral maturation was therefore investigated in 22 baboon thoracic specimens to elucidate its relationship with biomechanics and volumetric bone mineral density (vBMD). Cadaveric baboon vertebrae were used due to the limited availability of human tissues in the pediatric age range. The specimen ages ranged between 1 and 30 human-equivalent years based on skeletal maturity. Isolated ninth thoracic vertebrae (T9) were subjected to compressive loading to document their compressive mechanical properties (yield load, stiffness, yield strength, and elastic modulus) and ashed to determine their volumetric bone mineral density. Spinal maturation was discovered to significantly increase vBMD (P < 0.0001) and compressive mechanics (stiffness, bulk elastic modulus, failure load, and bulk strength, P < 0.001) in a sex-dependent manner. Vertebral stiffness increased from 1218 N/mm at 1 year to 3534 N/mm at 30 years with a second order polynomial "maturation" relationship. Volumetric bone mineral density and vertebral cross-sectional area together described the developmental patterns of stiffness and yield load of isolated vertebrae. Sex differences were observed throughout development, demonstrating differing growth patterns to accommodate mechanical loading whereby males develop larger size vertebrae and females achieve their mechanical stiffness and strength through greater bone mineral density.

Preclinical evaluation of a poly (vinyl alcohol) hydrogel implant as a replacement for the nucleus pulposus

STUDY DESIGN

An in vivo investigation into the safety of a novel hydrogel implant designed to replace the diseased nucleus pulposus.

OBJECTIVES

To determine the local and systemic safety of this new implant in a nonhuman primate model.

SUMMARY OF BACKGROUND DATA

A poly (vinyl alcohol) (PVA) hydrogel has been developed as a prosthetic replacement for the diseased nucleus pulposus.

METHODS

PVA implants were inserted into discectomy defects created in the L3-L4 or L4-L5 intervertebral disc in 20 male baboons. Empty discectomy defects served as a surgical control in 8 additional animals. Routine follow-up evaluations included radiography, magnetic resonance imaging, gross pathology, and histopathology of both local and remote tissues.

RESULTS

Insertion of the PVA hydrogel from an anterior direction produced extrusions in 5 animals from the first series of 15 surgeries (33%). A modified surgical technique, involving an anterolateral rather than anterior approach, was used in 5 animals, but the extrusion rate remained high (20%). Despite these surgical complications, the PVA implants were well tolerated over 24 months in vivo, with no evidence of device-related pathology in the adjacent disc tissue, spinal cord, or remote tissues.

CONCLUSION

Implantation of the PVA implant for periods of up to 24 months produced no evidence of local or systemic toxicity. Additional studies are now needed to determine the efficacy of the device in its intended application.

Compressive tolerance of the maturing cervical spine

While a number of experiments have reported injury thresholds for the adult cervical spine in compression, the compressive failure tolerance of the child spine has not been characterized. In order to develop useful safety measures for children, the biomechanical effects of maturation must be evaluated. Hence, this study examined the effects of spinal development on the compressive mechanics of the cervical spine. An animal model was used due to the lack of human tissues in the pediatric age range. Twenty-two fresh cadaveric baboon cervical spines (all male) were dissected into two functional spinal unit segments: Occiput-C2, C3-C5, C6-T1. The specimens ranged in age from 1 to 30-human equivalent years based upon radiographic assessment of their skeletal maturity. Dynamic (1.0-m/sec) haversine displacement inputs up to 70% strain were imparted on each specimen and the resulting loads were recorded. Significant increases in the compressive failure load were observed with increased maturation of the spinal tissues (ANOVA, p = 0.003). Differences were also observed between the spinal levels examined. The lower cervical spine, C6-T1, had the smallest failure load for specimens greater than 8-human-equivalent years, while the upper cervical spine was the most susceptible to injury at less than 8-human-equivalent years. The compressive failures generated are consistent with those observed clinically, consisting of primarily burst fractures and physis (growth plate) failures. These data clearly suggest that spinal compressive tolerance is directly related to maturation. Therefore, through scaling, these data may provide tolerance values applicable to anthropomorphic test dummies and computational models aimed at injury prevention for the pediatric occupant.

Incidence, epidemiology, and occupational outcomes of thoracolumbar fractures among U.S. Army aviators

BACKGROUND

The routine occupational hazards of flying and parachute jumping place U.S. Army aviators at risk for sustaining high-energy traumatic injuries, such as thoracolumbar fractures.

METHODS

A longitudinal, prospective, epidemiologic database was used to determine the incidence, injury history, and aeromedical disposition of U.S. Army aviators who sustained thoracolumbar fractures for calendar years 1987 to 1997.

RESULTS

The overall incidence rate of thoracolumbar fracture was 12.8 per 100,000 aviators per year. Thirty aviators with thoracolumbar fractures were identified, and the average age at time of injury was 35.9 years (range, 25-59 years). Mean follow-up after injury was 6.5 years (range, 2-12 years). Helicopter crashes and parachuting accidents accounted for 73% of fractures. Neurologic injury occurred in 10% of aviators. Seventy-seven percent of injured aviators recovered sufficiently to return to aviation service. There was no association between type of treatment and eventual termination from aviation duties (relative risk, 1.1; 95% confidence interval, 0.7-1.6).

CONCLUSION

Occupational hazards of Army aviators place them at risk for sustaining thoracolumbar fractures. These data are relevant to future decisions for research and resource allocation for aviation safety and policy.

Cervical spine models for biomechanical research

Biomechanical models have been used for the understanding of the basic normal function and dysfunction of the cervical spine and for testing implants and devices. Biomechanical models can be broadly categorized into four groups: 1) Physical models, made of nonanatomic material (e.g., plastic blocks), are often used for the testing of spinal instrumentation when only the device is to be evaluated. 2) In vitro models consisting of a cadaveric spine specimen are useful in providing basic understanding of the functioning of the spine. Human specimens are more suitable for these models than are animal specimens whenever anatomy, size (for instrumentation), and kinematics are important. Animal specimens are less costly, easier to obtain, and often have less variability but should be used with care because of the absence of anatomic fidelity with the human. 3) In vivo animal models provide the means to model living phenomena, such as fusion, development of disc degeneration, instability, and adaptive responses in segments adjacent to spinal instrumentation. Choosing the appropriate animal is important. The appropriate animal should have spinal loading, kinematics, kinetics, vertebral size, and healing-fusion rates as similar to those in humans as possible. For better interpretation of in vivo animal experimental results, in vitro biomechanical study using the same animal cadaveric specimen is useful but has not been used routinely. 4) Computer models are developed from mathematical equations that incorporate geometry and physical characteristics of the human spine and may be advantageously used for problems that are difficult to model by other means. Examples are the changes in disc and vertebral stresses in response to graded transection of facet joints and the study of changes in endplate loading caused by disc degeneration. Because these models are purely mathematical, their validation is essential. Validation is best achieved by first incorporating high-quality geometry and physical characteristics of the human spine and then comparing the model predictions with experimental observations. Sometimes an enthusiastic researcher may use a computer model beyond its validation boundary, making the model's predictions unreliable. In general, it is important to remember that a biomechanical model, similar to any other model, represents only a certain aspect of the real living human being. The aspect chosen for representation should be selected with great care. The model should be designed to answer specifically the question asked. Its predictions are valid only within the boundaries of assumptions and limitations that it incorporates.

Biomechanical effects of transoral odontoidectomy

The acute biomechanical effects of transoral odontoidectomy were studied by using qualitative and quantitative methods to assess atlantoaxial motion. In vitro biomechanical testing was performed on the upper cervical spines of eight baboon and five human cadaveric specimens. Using an unconstrained testing apparatus, we performed a flexibility method of testing. Physiological range loading was applied to atlantoaxial specimens, and three-dimensional motion was analyzed with stereophotogrammetry. Force-deformation relationships were delineated in intact specimens and again after surgical removal of the anterior C1 arch, odontoid process, and transverse atlantal ligament. We studied the total range of rotational and linear motions, the behavior of the neutral zone and elastic zone, the flexibility coefficients, and the instantaneous axes of rotation during flexion, extension, bilateral lateral bending, and bilateral axial rotation. Odontoidectomy produced several distinct alterations in motion and in force-deformation responses at C1-C2 that were almost identical in the baboon and human specimens. After odontoidectomy, the atlas developed significantly increased translational movements, which were most prominent in the anteroposterior direction. The total angular range of motion increased significantly during flexion, extension, and lateral bending but not during axial rotation. When the total range of motion was altered, the neutral zone was affected selectively and the elastic zone was spared. Surgery produced mobile, widely spread, unconstrained instantaneous axes of rotation that were in a constrained, fixed position in intact specimens. Clinically, transoral odontoidectomy may predispose patients to spinal instability. Even if acute spinal instability is not apparent, the patients may be susceptible to the delayed effects of the surgery because of the altered anatomy and biomechanical responses.