A kinematic and anthropometric study of the upper cervical spine and the occipital condyles.

The center of rotation (COR) of the upper cervical spine (UCS) is an important biomechanical landmark that is used to determine upper neck moment, particularly when evaluating injury risk in the automotive environment. However, neither the location of the UCS CORs nor the occipital condyles (OCs), which are frequently the referenced landmark for UCS CORs, have been measured with respect to known cranial landmarks. This study determines the CORs using pure bending (+/-3.5 N m), 3D digitization, and image analysis. Landmarks digitized included the OCs, external auditory meatus (EAM), infraorbital foramen, zygion, nasion, and the foramen magnum. The centroid of each occipital condylar surface (area 301+/-29.8 mm(2); length 25.4+/-3.2 mm) was located 18.4 mm posterior, 54.4 mm medial, and 31.0 mm inferior of the EAM. The UCS CORs were distinct: On average, OC-C1 CORs (22.5 mm posterior and 22.6 mm inferior to the left EAM) were superior and more posterior of OCs; C1-C2 CORs (7.4 mm posterior and 46.7 mm inferior to the left EAM) were inferior and more anterior of OC; and OC-C2 CORs (17.0 mm posterior and 33.1 mm inferior to the left EAM) were aligned with OC. There was a statistically significant difference between the percentage of UCS rotation in C1-C2 and OC-C1; 45% of the flexion and 71% of the extension occurred in OC-C1. Details of an anatomical variant with two pairs of distinct condylar surfaces are also presented.

Flexion and extension structural properties and strengths for male cervical spine segments.

New vehicle safety standards are designed to limit the amount of neck tension and extension seen by out-of-position motor vehicle occupants during airbag deployments. The criteria used to assess airbag injury risk are currently based on volunteer data and animal studies due to a lack of bending tolerance data for the adult cervical spine. This study provides quantitative data on the flexion-extension bending properties and strength on the male cervical spine, and tests the hypothesis that the male is stronger than the female in pure bending. An additional objective is to determine if there are significant differences in stiffness and strength between the male upper and lower cervical spine. Pure-moment flexibility and failure testing was conducted on 41 male spinal segments (O-C2, C4-C5, C6-C7) in a pure-moment test frame and the results were compared with a previous study of females. Failures were conducted at approximately 90 N-m/s. In extension, the male upper cervical spine (O-C2) fails at a moment of 49.5 (s.d. 17.6)N-m and at an angle of 42.4 degrees (s.d. 8.0 degrees). In flexion, the mean moment at failure is 39.0 (s.d. 6.3 degrees) N-m and an angle of 58.7 degrees (s.d. 5.1 degrees). The difference in strength between flexion and extension is not statistically significant. The difference in the angles is statistically significant. The upper cervical spine was significantly stronger than the lower cervical spine in both flexion and extension. The male upper cervical spine was significantly stiffer than the female and significantly stronger than the female in flexion. Odontoid fractures were the most common injury produced in extension, suggesting a tensile mechanism due to tensile loads in the odontoid ligamentous complex.

The human cervical spine in tension: effects of frame and fixation compliance on structural responses.

There is little data available on the responses of the human cervical spine to tensile loading. Such tests are mechanistically and technically challenging due to the variety of end conditions that need to be imposed and the difficulty of strong specimen fixation. As a result, spine specimens need to be tested using fairly complex, and potentially compliant, apparati in order to fully characterize the mechanical responses of each specimen. This, combined with the relatively high stiffness of human spine specimens, can result in errors in stiffness calculations. In this study, 18 specimen preparations were tested in tension. Tests were performed on whole cervical spines and on spine segments. On average, the linear stiffness of the segment preparations was 257 N/mm, and the stiffness of the whole cervical spine was 48 N/mm. The test frame was found to have a stiffness of 933 N/mm. Assembling a whole spine from a series combination of eight segments with a stiffness of 257 N/mm results in an estimated whole spine stiffness of 32.1 N/mm (32% error). The segment stiffnesses were corrected by assuming that the segment preparation stiffness is a series combination of the stiffnesses of the segment and the frame. This resulted in an average corrected segment stiffness of 356 N/mm. Taking the frame compliance into account, the whole spine stiffness is 51 N/mm. A series combination of eight segments using the corrected stiffnesses results in an estimated whole spine stiffness of 45.0 N/mm (12% error). We report both linear and nonlinear stiffness models for male spines and conclude that the compliance of the frame and the fixation must be quantified in all tension studies of spinal segments. Further, reported stiffness should be adjusted to account for frame and fixation compliance.

A comparison of stainless steel and CP titanium rods for the anterior instrumentation of scoliosis.

Use of 4.0 mm and 5.0 mm steel rods have resulted in proximal screw pullout. Titanium rods, which encompass a reduced yield point, may increase the effective stiffness of the construct when used with segmental anchors. Seven human thoracic spines were loaded in axial compression, axial torsion, flexion, extension, and lateral bending. Testing was performed on intact, discectomy and simulated bony fusion specimens. Specimens were randomly instrumented with 4.0 mm and 5.0 mm steel, and 4.5 mm CP Ti rods. In compression and torsion, the intact spine demonstrated increased stiffness with respect to all instrumentation employed in a discectomy condition. No significant differences between the intact and the instrumentation systems were detected for torsion or compression under simulation of fusion. Under flexural loading, no significant differences were detected between the intact specimen and the instrumentation systems. When used in conjunction with segmental force anchors, the use of CPTi rods which posses reduced yield points can provide sufficient rigidity as compared to stainless steel rods of comparable diameter. Implants of reduced yield point will permit permanent deformation with low force. Screw pullout may result when high yield materials are employed in conjunction with anchors.

Ultra high molecular weight polyethylene (UHMWPE) model can provide results comparable to cadaveric models.

The in vitro biomechanical models using a cadaveric spine specimen have long been used in understanding normal and abnormal functions of spines as well as for strength and stability testing of the spine specimen or spinal construct. Little effort has been made to describe the similarities or differences between UHMWPE and cadaveric models. Eight cadaveric lumbar spines were harvested generating six FSU and three corpectomy models. Six UHMWPE blocks were fabricated to form FSU and corpectomy models. All were tested intact, with posterior instrumentation, and with anterior instrumentation consisting of Moss-Miami 4.0 mm stainless steel rods, uni-axial stainless steel screws and DePuy Harm's cages. All models were tested in axial compression. The cadaveric model and UHMPWE model yielded axial stiffness values of comparable magnitude with respect to instrumentation applied using the posterior approach (P>0.05). Under an FSU configuration, only in the case of anterior instrumentation without the addition of a Harm's cage did both the cadaveric and UHMPWE models provide comparable axial stiffness results (P>0.05). While in vitro cadaveric models are considered the gold standard for biomechanical testing of the spine, the data suggests that under specific approaches and surgical models UHMWPE can be used to infer mechanical performance of instrumentation in cadaveric material.

Position of hip resurfacing component affects strain and resistance to fracture in the femoral neck.

BACKGROUND: Retrieval studies have suggested that the cause of femoral implant failure after metal-on-metal hip resurfacing is multifactorial. Both varus positioning of the femoral component and notching of the superior part of the femoral neck have been associated with femoral component failure. The hypotheses of this study were that placement of a femoral resurfacing component alters femoral neck loading and that the cortical strain pattern reflecting this loading is directly related to the spatial orientation of the resurfacing component. An additional hypothesis was that notching of the superior part of the neck during implantation results in a decreased resistance to neck fracture under axial loading. METHODS: Varus, anteverted, retroverted, and anatomic positions of the femoral component were tested in sixty-four cadaveric femora. Simulated stance-phase loading was applied, and the shear strain on the femoral neck cortex was quantified with use of a photoelastic method. Preimplantation and postimplantation strain levels were compared over the entire neck region with use of generalized estimating equations. The influence of anteversion and retroversion of the component and notching of the superior part of the neck on the neck strength were evaluated. RESULTS: Placement of the implant in 10 degrees of varus alignment relative to anatomic positioning increased strain on the superior aspect of the neck by 19% to 23% compared with intact femora. Anteverted and retroverted placement of the implant produced elevated strain in the anterior-inferior and posterior-inferior aspects of the neck, respectively. Placement of the component stem in alignment with the anatomic neck axis decreased neck cortical strain 6% to 19% compared with intact femora. Notching of the superior aspect of the neck decreased neck strength by 21%. CONCLUSIONS: Relatively small deviations from anatomic alignment of a resurfacing hip component result in marked localized increases in loading of the femoral neck under conditions approximating single-limb stance. Neutral positioning of the femoral component results in localized strain reduction. Notching of the superior aspect of the femoral neck significantly reduces the resistance to fracture (p = 0.008).

Comparative structural neck responses of the THOR-NT, Hybrid III, and human in combined tension-bending and pure bending.

This study evaluated the biofidelity of both the Hybrid III and the THOR-NT anthropomorphic test device (ATD) necks in quasistatic tension-bending and pure-bending by comparing the responses of both the ATDs with results from validated computational models of the living human neck. This model was developed using post-mortem human surrogate (PMHS) osteoligamentous response corridors with effective musculature added (Chancey, 2005). Each ATD was tested using a variety of end-conditions to create the tension-bending loads. The results were compared using absolute difference, RMS difference, and normalized difference metrics. The THOR-NT was tested both with and without muscle cables. The THOR-NT was also tested with and without the central safety cable to test the effect of the cable on the behavior of the ATD. The Hybrid III was stiffer than the model for all tension-bending end conditions. Quantitative measurement of the differences in response showed more close agreement between the THOR-NT and the model than the Hybrid III and the model. By contrast, no systematic differences were observed in the head kinematics. The muscle cables significantly stiffened the THOR-NT by effectively reducing the laxity from the occipital condyle (OC) joint. The cables also shielded the OC upper neck load cell from a significant portion of the applied loads. The center safety significantly stiffened the response and decreased the fidelity, particularly in modes of loading in which tensile forces were large and bending moments small. This study compares ATD responses to computational models in which the models include PMHS response corridors while correcting for problems associated with cadaveric muscle. While controversial and requiring considerable diligence, these kinds of approaches show promise in assessing ATD biofidelity.

Biomechanical analysis of anterior poly-methyl-methacrylate reconstruction following total spondylectomy for metastatic disease.

STUDY DESIGN: Three reconstruction options were evaluated biomechanically following total spondylectomy using human cadaveric spine specimens. OBJECTIVES.: To evaluate and compare the stability of combined anterior and posterior fixation incorporating poly-methyl-methacrylate with alternative accepted reconstruction techniques. SUMMARY OF BACKGROUND DATA: Total spondylectomy represents the most radical option for decompression in metastatic spinal cord compression. Poly-methyl-methacrylate is considered a useful adjunct in spinal column stabilization and arthrodesis; however, there is little published biomechanical data to support its use in this setting. METHODS: Ten fresh-frozen human cadaveric spines (T9-L3) were used. After intact analysis, a total spondylectomy was performed at T12. Three potential reconstruction techniques were tested for their ability to restore stiffness to the specimen: 1) multilevel posterior pedicle screw instrumentation from T10-L2; 2) anterior instrumentation (ATL Z plate II) and rib graft at T11-L1 with multilevel posterior pedicle screw instrumentation from T10-L2; and 3) anterior cement (Simplex P) and pins construct (T12) with multilevel posterior pedicle screw instrumentation from T10-L2. Each of the three potential reconstruction techniques was tested on each specimen in random order using nondestructive testing under load control. RESULTS: Only combined stabilization techniques (e.g., anterior instrumentation and rib graft with multilevel posterior pedicle screw instrumentation and anterior cement-and-pins construct with multilevel posterior pedicle screw instrumentation) restored stiffness to a level equivalent to or higher than that of the intact spine in all loading modes (P < 0.05). Anterior cement-and-pins construct with multilevel posterior pedicle screw instrumentation provided more stability to the specimen than anterior instrumentation and rib graft with multilevel posterior pedicle screw instrumentation in compression and flexion testing (P < 0.05). Posterior instrumentation alone did not restore stiffness to the intact level in compression and flexion testing (P < 0.005). CONCLUSIONS: Combined anterior and posterior reconstruction using a cement construct provides equal to or more stability than the intact spine in all testing modes. Posterior stabilization alone is an inferior method of reconstruction following total spondylectomy. Poly-methyl-methacrylate has the advantage over traditional anterior reconstruction techniques in that it can be inserted using a posterior approach.