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Arthroplasty, Replacement↗
Biomedical subjects
Bryan Cunningham
Publications and source records attributed to Bryan Cunningham.
Cervical disc replacement-porous coated motion prosthesis: a comparative biomechanical analysis showing the key role of the posterior longitudinal ligament.
STUDY DESIGN: Benchtop cadaveric biomechanical comparative testing and caprine animal model in vivo implantation. OBJECTIVE: To evaluate the role of the posterior longitudinal ligament in cervical arthroplasty and to understand the relative contribution of this ligament in nonfusion applications. SUMMARY OF BACKGROUND DATA: Rauschning refers to the posterior longitudinal ligament as "The Kleenex Ligament" due to its apparent anatomic insignificance. White and Panjabi found the posterior longitudinal ligament ranked only fourth in importance in tensile load-to-failure biomechanical testing. In the postoperative situation following anterior cervical diskectomy fusion, posterior longitudinal ligament integrity is overlooked by physicians because the entire disc space usually fuses into a homogeneous block of bone. PURPOSE: This biomechanical study was undertaken to determine the relative importance of the posterior longitudinal ligament following two different degrees of anterior decompression, anterior disc replacement, and anterior arthrodesis procedures. METHODS: A total of seven fresh frozen human cadaveric cervical spines (C3-C7) (mean age 68 +/- 19 years) were used for biomechanical testing. Each vertebra was equipped with three non-colinear light emitting diodes designed for detection by an optoelectronic motion measurement system (3020 Optotract System). To determine the multidirectional flexibility, six pure moments (flexion, extension, right + left lateral bending, right + left axial rotation) and axial compression were applied using a servohydraulic 858 Bionix testing device configured with a six-degree-of-freedom spine simulator. Range of motion was defined as the peak displacement from the initial neutral position to the maximum load, whereas the neutral zone represents the motion from the initial neutral position to the unloaded position at the beginning of the third cycle. Seven groups of (N = 7 each) constructs at C5-C6 were: 1) intact "native" C5-C6 level; 2) anterior diskectomy (posterior longitudinal ligament intact); 3) a Low Profile Porous Coated Motion cervical disc replacement; 4) posterior longitudinal ligament resected; 5) Porous Coated Motion cervical disc replacement fixed with anterior flanges and screws; 6) tricortical structural allograft; and 7) an anterior cervical translational plate + allograft. The caprine model was evaluated for suitability as an animal model with 12 goats undergoing C3-C4 anterior cervical Porous Coated Motion disc replacement. RESULTS: Group 2 (anterior diskectomy alone) was significantly more stable than Group 4 (anterior diskectomy + posterior longitudinal ligament resection) in flexion-extension, 18.7 +/- 4.76 degrees versus 24.8 +/- 4.42 degrees (P < 0.05) and in lateral bending, 5.9 +/- 1.79 degrees versus 10.7 +/- 2.8 degrees (P < 0.05). The comparison for the two conditions for axial rotation, 10.4 +/- 13.9 degrees versus 13.9 +/- 2.7 degrees, and axial compression, 1.19 +/-.98 degrees versus 1.52 +/- 1.14 degrees, showed the same trend. Twelve goats undergoing porous coated motion cervical disc replacement had no evidence of prosthesis loosening, neurologic complications, or experienced inflammatory reactions from particulate wear debris after 6 months of implantation. DISCUSSION: This study confirms the pivotal role of the posterior longitudinal ligament in postsurgical stability of the cervical spine following anterior diskectomy. This is because the lateral anulus, uncovertebral ligaments, and lateral capsular ligaments are stretched and plastically deformed in the surgical distraction process of restoring the disc space height following anterior surgical decompression. There should be a separate determination of the range of motion of cervical disc replacements depending of the integrity and the amount of the posterior longitudinal ligament that has been resected. CLINICAL RELEVANCE: There are two basic types of total knee replacements, posterior cruciate ligament-preserving and posterior cruciate ligament-sacrificing designs. In the cervical spine, an analogous situation exists biomechanically depending on whether the posterior longitudinal ligament needs to be removed in its entirety as part of the spinal cord decompression part of the procedure--it may be helpful to conceptually differentiate between posterior longitudinal ligament-preserving and posterior longitudinal ligament-sacrificing total cervical disc replacements.
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Preclinical testing of a wedge-rod system for fusionless correction of scoliosis.
STUDY DESIGN: Biomechanical study. OBJECTIVES: This paper describes the biomechanical comparison of calf spines instrumented with a wedge alone versus a wedge-rod construct for the fusionless correction of scoliosis. SUMMARY OF BACKGROUND DATA: Current techniques for the correction of scoliosis require either anterior or posterior spinal fusion for correction. We propose a technique allowing correction via vertebral body osteotomies along with fixation using a wedge-rod construct without the requirement of intervertebral segment fusion. METHODS: Calf spines were used to test the biomechanical difference between the intact spine, transverse osteotomized spine with wedge-rod reconstruction, and transverse osteotomized spine with wedge alone reconstruction. Unconstrained segments (L1-L5) were first tested under five nondestructive static loading conditions to evaluate the intact stability of the operative motion segments in axial compression (-600 N), axial rotation (+/-5.0 Nm, 150 axial preload), flexion and extension (+/-5.0 Nm), and lateral bending (+/-5.0 Nm). Following the intact analysis, vertebral wedge osteotomies in the transverse plane were performed at the L2, L3, and L4 levels. The defects were reconstructed using the Sofamor Danek Wedge Spacer, and stainless steel TSRH one-quarter inch single rod with modified CD HORIZON 6.5-mm diameter vertebral body screws at each level (L2-L4). Standard CD HORIZON 6.5-mm bone screws and staples were used at the superior and inferior ends of the five-level construct. The wedge was on the left side and the rod and screw heads on the right side. After testing the reconstructed specimen, the TSRH rod was removed and the construct retested to evaluate the stability of the wedge alone reconstruction. RESULTS: Construct stiffness was calculated as the peak applied load (N or Nm) divided by the corresponding segmental displacement (mm or degrees) normalized to the intact specimen. Reconstruction static data are expressed as a percentage change from the intact condition. Statistical analysis included descriptives, a one-way analysis of variance, and the Student-Newman-Keuls test for multiple comparisons among the reconstruction groups. Axial compression: under axial compressive loads, the stiffness of the wedge-rod construct was approximately equal to that of the intact group. The stiffness of the wedge alone construct was 56% less than that of the intact group except for extension and left lateral bending. For the other modes of loading (right rotation, left rotation, flexion, extension, right lateral bending, and left lateral bending), the wedge-rod construct was stiffer than that of the intact group. The stiffness of the wedge alone construct was consistently less than that of the intact group. CONCLUSIONS: Based on the results of this biomechanical comparison, the calf spines instrumented with the wedge-rod system for fusionless correction were significantly stiffer as compared to the intact calf spine. The wedges alone were not as stiff as the intact spine. This suggests that the theory of performing transverse osteotomies of vertebral bodies with fixation with wedge-rod construct for 8 to 12 weeks, followed by removal of the rod, could provide adequate fixation and correction of a scoliotic deformity without requiring fusion of motion segments.
Animals↗
Straight-forward versus anatomic trajectory technique of thoracic pedicle screw fixation: a biomechanical analysis.
STUDY DESIGN: A biomechanical study on cadaveric thoracic vertebrae using pullout strength, insertional torque, and bone mineral density to determine the optimal sagittal trajectory of thoracic pedicle screws. OBJECTIVE: To perform a biomechanical study on cadaveric thoracic vertebrae using insertional torque, pullout strength, and bone mineral density to determine the optimal biomechanical sagittal trajectory for placement thoracic pedicle screws. We compared the straight-forward (paralleling the vertebral endplate) with anatomic trajectory (directed along the true anatomic axis of the pedicle). METHODS: Thirty cadaveric thoracic vertebrae were harvested and evaluated with dual-energy x-ray absorptiometry to assess bone mineral density. Matched, fixed-head pedicle screws were then randomly assigned by side and placed using the straight-forward or anatomic technique under fluoroscopic visualization while recording the maximum insertional torque. Pullout strength testing was then performed. RESULTS: The maximum insertional torque for the straight-forward technique was 2.58 +/- 0.14 (SE) in pounds, whereas the anatomic technique averaged 1.86 +/- 0.14 (SE) in pounds (P = 0.0005). The maximum insertional torque at the neurocentral junction for the straight-forward technique averaged 1.89 +/- 0.17 (SE) in-lbs. (73% of maximum insertional torque), whereas the anatomic trajectory averaged 1.39 +/- 0.11 (SE) in pounds (75% of maximum insertional torque) (P = 0.007). The average pullout strength using a straight-forward trajectory was 611 +/- 50 (SE) N compared to the anatomic trajectory, which averaged 481 +/- 54 (SE) N (P = 0.034). The pullout strength correlated with mean bone mineral density for both the straight-forward (r = 0.461, P = 0.027) and anatomic (r = 0.598, P = 0.004) techniques. CONCLUSIONS: The straight-forward technique results in a 39% increase in maximum insertional torque and a 27% increase in pullout strength compared to the anatomic technique. The maximum insertional torque at the neurocentral junction resulted in a 36% increase using the straight-forward technique versus the anatomic trajectory. Bone mineral density directly correlates with pullout strength for both techniques.
Absorptiometry, Photon↗
Braided hamstring tendons for reconstruction of the anterior cruciate ligament. A biomechanical analysis.
BACKGROUND: In an effort to improve the strength and stiffness of anterior cruciate ligament grafts, several authors have advocated alterations of graft structure and orientation, including braiding the tendons in hamstring tendon grafts. HYPOTHESIS: Braiding hamstring tendons does not increase graft strength and stiffness. STUDY DESIGN: Controlled laboratory study. METHODS: Sixteen hamstring tendon and 21 bone-patellar tendon-bone grafts were harvested from 12 cadavers and divided into three groups: 1) braided four-strand hamstring tendon, 2) unbraided four-strand hamstring tendon, and 3) bone-patellar tendon-bone. All grafts were placed under a 50-N preload on a servohydraulic testing device and were tensioned to failure. RESULTS: The strength and stiffness of the tested specimens averaged 427 +/- 36 N and 76 +/- 10 N/mm, respectively, for braided specimens, 532 +/- 44 N and 139 +/- 18 N/mm for unbraided specimens, and 574 +/- 46 N and 158 +/- 15 N/mm for patellar tendon specimens. There was a 20% decrement in hamstring tendon graft tensile strength and a 45% decrease in stiffness after braiding because of the suboptimal multidirectional orientation of individual tendons within the braided grafts. CONCLUSIONS: In vitro braided hamstring tendon grafts demonstrated mechanically inferior strength and stiffness characteristics compared with unbraided hamstring tendon grafts and patellar tendon grafts. CLINICAL RELEVANCE: Braiding of hamstring tendon grafts provides no mechanical advantage in anterior cruciate ligament reconstruction.
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