Biomechanical Comparison of Two Surgical Repair Techniques of the Distal Biceps Tendon.

Purpose: This study aimed to characterize the relationship between the distal biceps tendon force and the supination and flexion rotations during the initiation phase and to compare the functional efficiency of anatomic versus nonanatomic repairs. Methods: Seven matched pairs of fresh-frozen cadaver arms were dissected to expose the humerus and elbow while preserving the biceps brachii, elbow joint capsule, and distal radioulnar soft tissue complex. For each pair, the distal biceps tendon was severed with a scalpel and then repaired with bone tunnels placed at either the anterior (anatomic) or the posterior (nonanatomic) aspect of the bicipital tuberosity on the proximal radius. A supination test with 90° of elbow flexion and an unconstrained flexion test were conducted on a customized loading frame. The biceps tension was applied incrementally at 200 g per step, whereas the radius rotation was tracked with a 3-dimensional motion analysis system. The tendon force needed to produce a degree of supination or flexion was derived as the regression slope of the tendon force-radial rotation plots. A two-tailed paired t test was performed to compare the difference between the anatomic repair and the nonanatomic repair cadavers. Results: Significantly greater tendon force was required to initiate the first 10° of supination with the elbow in flexion for the nonanatomic group compared with the anatomic group (1.04 ± 0.44 N/degree vs 0.68 ± 0.17 N/degree, P = .02). The average nonanatomic to anatomic ratio was 149% ± 38%. No difference existed between the two groups in the mean tendon force needed to produce the degree of flexion. Conclusions: Our results show that anatomic repair is more efficient in producing supination than nonanatomic repair, but only when the elbow is in 90° of flexion. When the elbow joint is not constrained, the nonanatomic supination efficiency improved, and the difference between the techniques was not significant. Clinical relevance: The present study added to the body of evidence in comparing anatomic versus nonanatomic repair of the distal biceps tendon and serves as a foundation for future biomechanical and clinical studies in this topic. Given no difference when the elbow joint was not constrained, one could argue that surgeon comfort and preference could guide which technique to use when addressing the distal biceps tendon tears. More studies will be needed to clearly define whether there will be a clinical difference between the two techniques.

Biomechanical Evaluation of Transverse Patella Fracture Fixation Using Headless Screws: A Comparison of Suture Versus Cable as a Tension Band.

OBJECTIVES: To explore the performance of headless screws with FiberWire Suture as a tension band and headless screws with a mini-cable tension band in patella fixation. METHODS: A transverse osteotomy was created in 6 matched pairs of fresh-frozen cadaver knee joints. One knee was randomly assigned to receive fixation with headless screws plus a cable tension band while the other was fixed with headless screws plus a suture tension band. Using a servo-hydraulic material testing system, the specimens were first tested nondestructively under 20% of the reported mean failure load with a standard technique of cannulated screws with tension band wiring. The specimen was then loaded to 1000 N to test the construct's failure strength. All tests were run under displacement-control with loading threshold. A motion analysis system was used to track the interfragmentary motion to assess fixation stability. RESULTS: In the nondestructive loading test, gap displacement under 150 N was 0.10 mm or less for 11 of 12 specimens, and the difference between the 2 groups was not statistically significant. In the destructive test, 3 of 12 specimens maintained reduction (gap <2 mm) at the maximum load of 1000 N. Of the failed specimens, the mean strength was 648 ± 185 N for suture and 784 ± 228 N for cable. CONCLUSIONS: There was no significant difference in fixation strength or subfailure fragment displacement between the suture and cable tension band techniques when using headless screws.

Effect of age on femur whole-bone bending strength of mature rat.

BACKGROUND: Skeletally mature rodents are frequently used in studies of bone health and bone healing, some of them requiring longitudinal observations that span a significant portion of the animals' adulthood. However, changes in whole bone mechanics associated with the natural aging of adult rats have not been extensively characterized. METHODS: Femurs from skeletally mature Wistar rats in three age groups of 24-week (young adult), 39-week (middle-age), and 54-week (late middle-age) were tested under three-point bending load in the anterior-posterior direction. Mechanical properties and geometric properties of the femurs from the two older groups were compared to the 24-week rats. FINDINGS: Significantly greater strength, rigidity, and post-yield deformation were found in the 54-week group when compared to the 24-week group. The oldest group also demonstrated greater leg length, anteroposterior width, and cross-sectional moment of inertia over the youngest group. Of the intrinsic properties, the highest ultimate stress was found in the 39-week and was significantly higher than the 24-week group. The ultimate strain increased with age, and the difference between the youngest and the oldest group was statistically significant. INTERPRETATION: The results suggest that femoral bending properties and geometric properties are continually modified from young adult to late-middle-aged animals. Knowing the baseline bone strength and rigidity throughout adulthood of a rodent breed helps guide animal selection in study design.

Biomechanical Comparison of Tension Band Fixation of Patella Transverse Fracture: Headless Screws Versus Headed Screws.

OBJECTIVE: This study aimed to investigate the stability and strength of tension band wire fixation using headless compression screws versus headed screws for transverse patella fractures. METHODS: Six matched pairs of fresh-frozen cadaveric knees with transverse osteotomies created at the midpoint of the patella were surgically fixed, with one knee randomly receiving fixation with headless screws (Acumed Acutrak 4/5) and the other with headed screws (Synthes 4.0 partially threaded cannulated screws). The specimens were mounted onto a servohydraulic load frame in a 45-degree flexed position and loaded through the quadriceps tendon. Interfragmentary movement was recorded with a motion analysis system. The initial fixation stiffness, range of interfragmentary motion, and strength of the headless screw construct were compared with the headed screw construct. Failure was defined as either a sudden drop in applied tendon force or 2 mm of separation on the anterior surface of the patella (ie, clinical failure), whichever occurred first. RESULTS: Mean primary interfragmentary motion was 0.31 ± 0.28 degrees for the headed screws and 0.10 ± 0.06 degrees for headless screws under 150 N load (P = 0.03). Mean construct stiffness was 277 ± 243 N/degrees for the headed screws and 510 ± 362 N/degrees for the headless screws (P = 0.03). None of the constructs from either group displayed structural failure before reaching the clinical failure gap of 2 mm. The mean clinical failure strength was 808 ± 183 N for the headless screws construct and 520 ± 241 N for the headed screws construct (P = 0.03). CONCLUSIONS: Headless screw tension band fixation demonstrated superior biomechanical behaviors over standard headed screw fixation with higher construct rigidity, smaller interfragmentary motion, and greater fixation strength.

Evaluation of cast creep occurring during simulated clubfoot correction.

The Ponseti method is a widely accepted and highly successful conservative treatment of pediatric clubfoot involving weekly manipulations and cast applications. Qualitative assessments have indicated the potential success of the technique with cast materials other than standard plaster of Paris. However, guidelines for clubfoot correction based on the mechanical response of these materials have yet to be investigated. The current study sought to characterize and compare the ability of three standard cast materials to maintain the Ponseti-corrected foot position by evaluating cast creep response. A dynamic cast testing device, built to model clubfoot correction, was wrapped in plaster of Paris, semi-rigid fiberglass, and rigid fiberglass. Three-dimensional motion responses to two joint stiffnesses were recorded. Rotational creep displacement and linearity of the limb-cast composite were analyzed. Minimal change in position over time was found for all materials. Among cast materials, the rotational creep displacement was significantly different (p < 0.0001). The most creep displacement occurred in the plaster of Paris (2.0°), then the semi-rigid fiberglass (1.0°), and then the rigid fiberglass (0.4°). Torque magnitude did not affect creep displacement response. Analysis of normalized rotation showed quasi-linear viscoelastic behavior. This study provided a mechanical evaluation of cast material performance as used for clubfoot correction. Creep displacement dependence on cast material and insensitivity to torque were discovered. This information may provide a quantitative and mechanical basis for future innovations for clubfoot care.

Biomechanical comparison of supplemental posterior fixations for two-level anterior lumbar interbody fusion.

Posterior instrumentations have been used to supplement anterior lumbar interbody fusion with cages. Biomechanical studies on single-level anterior lumbar interbody fusion show that stand-alone cages supplemented with posterior translaminar facet or transfacet screw fixation exhibit comparable stability to those supplemented with pedicle screw/rod fixation, while stability of multilevel anterior lumbar interbody fusion remains mostly unknown. The objectives of this study are to compare the stabilization of three supplemental posterior fixations to two-level anterior lumbar interbody fusion, including translaminar facet fixation, transfacet screw fixation, and pedicle screw/rod fixation. Flexibility tests were conducted on fresh-frozen calf spines with moment up to 8.5 N m in flexion, extension, lateral bending, and axial rotation. Each specimen was tested at three stages: intact, anterior lumbar interbody fusion using Polyetheretherketone (PEEK) interbody cage at L3-L4 and L4-L5, and the same anterior lumbar interbody fusion plus one of the three supplemental posterior fixations. The addition of the supplemental posterior fixation increased stiffness at the fusion levels significantly in flexion (9.9 times), extension (5.4 times), and lateral bending (4.1 times). The pedicle screw/rod and translaminar screw fixations provide approximately 40% higher stiffness than the transfacet screw in lateral bending. The pedicle screw/rod fixation also displayed a trend of superior fixation in extension. Supplemental posterior fixation significantly improved stability of two-level anterior lumbar interbody fusion when compared to the stand-alone cages. Pedicle screw/rod system is still the "gold standard" in providing supplemental stability. However, both translaminar facet screws and transfacet screws are good alternatives to provide adequate fixation.

Alteration of load sharing of anterior cervical implants with change in cervical sagittal alignment.

Anterior cervical discectomy and fusion (ACDF) is often supplemented with the application of an anterior plate to improve the stability of the fusion segment. While plate design has been shown to influence stress shielding of the graft, little is known about how the kyphotic alignment of a fused cervical segment affects the load sharing between the anterior plate and the osteoligamentous structures of the spine. The aim of this study was to characterize load sharing between an anterior plate and the osteoligamentous structures of the cervical motion segments in kyphotic versus normal lordotic alignment following single-level ACDF using fresh ovine cervical spines (C3-C6). The loading protocol involved preloading the spine with a 20 N compressive force and applying quasi-static moments (up to 2.1 Nm) in the sagittal plane to simulate flexion and extension. Stiffness of the fusion segment was measured from the moment-rotation plot. Normal lordotic alignment was replicated by insertion of a fibular allograft 2 mm taller than the interbody space. Kyphotic alignment was simulated by removing the graft and reapplying a shorter anterior cervical plate. The average segmental sagittal angulation at C4-C5 was 5.2+/-1.6 degrees of lordosis for the normal lordotic group and 6.8+/-2.3 degrees of kyphosis for the kyphotic group. With flexion, the plate shared 52.8% of the applied load in the normal lordotic group, and 70.1% in the kyphotic group (p<0.03). In extension, the amount of load-share by the plate in the normal lordotic group was comparable to that of the kyphotic group (52.7% vs. 40.7%, p=0.16). This study shows that kyphotic alignment of the cervical fusion segment increases the load sharing of the anterior plate under flexion loading.

Changes in the lumbar foramen following anterior interbody fusion with tapered or cylindrical cages.

BACKGROUND CONTEXT: Anterior lumbar interbody fusion (ALIF) using both cylindrical and tapered threaded interbody cages has been shown to restore disc height, reduce segmental motion, and relieve low back pain. The effectiveness of these stand-alone cage designs in restoration and maintenance of intervertebral foraminal dimensions has received little attention. PURPOSE: To investigate the effects of anterior implantation of cylindrical and tapered interbody cages on morphologic changes of the lumbar neuroforamen and maintenance of foraminal dimensions under dynamic loading. STUDY DESIGN/SETTING: A biomechanical study using bovine calf spine model to compare the deformation of foraminal space after ALIF with either tapered cages or cylindrical cages. METHODS: Sixteen fresh calf spines were randomly assigned to undergo ALIF at the L3-L4 level using either two threaded cylindrical or two tapered cages. Lumbar spines were subjected to unconstrained loading in flexion, extension, and lateral bending. Rotation of the L3-L4 segment and dynamic deformation in foraminal height were obtained through a motion analysis system, and compared between the two cage groups. Foraminal dimensions were assessed before and after tapered or cylindrical cage implantation with digitized measurement of bilateral foraminal molds. RESULTS: Regardless of cage design, anterior implantation of cages increased neuroforaminal area by 17% (p=.0005) and increased the foraminal height by 9% (p=.0004) in the neutral unloaded position. In dynamic loading conditions, foraminal height was significantly stabilized in all loading directions by the cylindrical cages (p=.01) and on both sides during lateral bending by the tapered cages (p<.03). Foraminal stabilization provided by either cage was most prominent in the direction of lateral bending (26-37% of the intact values), while cylindrical cages also provided substantial stabilization in flexion (26% of the intact value). Significant linear relationships were found between foraminal height and residual fusion segment motion under dynamic loading conditions. CONCLUSION: Results from this bovine model biomechanical study indicate that stand-alone anterior interbody fusion cages with either tapered or cylindrical design are effective in restoring neuroforaminal height and stabilize the spine to withstand foraminal deformation during daily loading. The degree of stabilization was influenced substantially by the loading direction, to a lesser degree by the cage type, and was strongly dependent on the segment mobility. Although bovine lumbar spine is widely accepted for comparative studies, direct clinical interpretation should be made with caution owing to the anatomical differences from human.

Biomechanical comparison of fixed-angle volar plate versus fixed-angle volar plate plus fragment-specific fixation in a cadaveric distal radius fracture model.

PURPOSE: To test the hypothesis that combining orthogonal fragment-specific fixation with volar fixed-angle fixation provides markedly higher interfragment stability and construct strength compared with volar fixed-angle fixation alone. METHODS: Eight matched pairs of fresh cadaveric hand and forearm specimens were potted upright in cement. Flexor and extensor tendons were isolated at insertion sites and sutured into a looped bundle for loading in flexion and extension, respectively (up to 61 N). Osteotomies to simulate an AO type C2, 3-part fracture pattern were created with a saw. One randomized specimen from each pair received a locking volar plate and a radial pin plate (VP+PP), and the other received a locking volar plate only (VP). The relative angular displacements between the radial, ulnar, and proximal fragments were obtained with a motion analysis system. After stability tests, specimens were compressed to failure in a wrist-extended position on a material testing machine. Paired t tests were used to compare the interfragment displacement, construct stiffness, and strength between the 2 groups. RESULTS: Comparing fragment displacement in the VP+PP and VP groups showed that with flexion-extension and radial-ulnar deviation, distal fragment displacement was reduced to a statistically significant degree. The VP+PP group also showed higher failure strength and construct rigidity than the VP group. CONCLUSIONS: In a simulated cadaveric model of the distal radius intra-articular fracture, the combined technique of fragment-specific plating with volar fixed-angle fixation alone provides superior biomechanical strength and stability over the volar fixed-angle fixation alone.

Does anterior plating of the cervical spine predispose to adjacent segment changes?

STUDY DESIGN: In a human cadaveric model, the effects of plate supplementation on the mechanical behaviors of adjacent segments were investigated. OBJECTIVES: The objective was to determine the effects of anterior cervical fusion and plating on the adjacent segments. SUMMARY OF BACKGROUND DATA: Increases in intradiscal pressure and intervertebral motion at adjacent segments have been reported in the lumbar spine following an instrumented fusion. It is unclear if the same phenomenon presents in the cervical spine. METHODS: Seven human cadaveric cervical spines (C2-T1) were used, and fusion of the C5-C6 segment was chosen for the purpose of this study. Two miniature pressure transducers were implanted within each adjacent disc. Flexion, extension, lateral bending, and torsion loads up to 2.5 Nm were applied to the intact spine, and following each of the two procedures, anterior discectomy and grafted fusion, and anterior plating of the C5-C6 motion segment. RESULTS: At the surgical level, a significant increase in segmental stiffness was observed after plating in all directions. Following the grafted fusion, there were no statistically significant changes at the superior adjacent segment, and there was a 13.7% increase in axial rotation in the inferior adjacent segment. Once anterior plating was applied, slight increase (<12%) over the intact spines was noted in lateral bending in both adjacent segments. However, there was no significant difference between the grafted fusion and anterior plated fusion at either adjacent segment. At both adjacent disc levels, the differences in intradiscal pressures between grafted fusion and plated fusion were less than 30% in all directions, and none of these differences was statistically significant. CONCLUSIONS: Intradiscal pressures and intervertebral motion at the adjacent levels are not significantly affected by the instrumented anterior fusion. The clinically observed degenerative change at adjacent segments in the cervical spine is more likely to be attributed to natural progression of the spondylotic process as opposed to biomechanical effect of the instrumentation or fusion.

Mechanical evaluation of posterior wiring as a supplement to anterior cervical plate fixation.

STUDY DESIGN: An in vitro experimental study was performed to examine 3-dimensional biomechanical stability of cervical fixations. OBJECTIVES: To determine whether posterior interspinous wiring contributes to the rigidity of a single-level motion segment that has been plated anteriorly, and to determine the effects of this combined fixation on intradiscal pressure and spinal motion at the adjacent segments. SUMMARY OF BACKGROUND DATA: Combined anterior and posterior column fixation is being increasingly used in a variety of clinical situations that do not involve complete disruption of the motion segment. The biomechanical validity of combined anterior posterior fixation in the absence of overt posterior ligamentous disruption has not been studied. METHODS: Six human fresh-frozen cadaveric cervical spines (C3-T1) were used. Three-dimensional intersegmental motion and intradiscal pressure were measured while the spine was loaded in flexion, extension, lateral bending, and torsion (up to 2.5 Nm). Fixation stability at the operative level (C5-C6) and influence of the fixation on adjacent segments were evaluated after an anterior plating procedure and combined anterior plating and posterior wiring. RESULTS: Comparing the combined approach with anterior plating alone, significant reductions in C5-C6 motion was noted: 49% in flexion (P <0.05), 48% in extension (P <0.003), and 33% and 39%, respectively, in left and right torsion (P <0.05). Reduction in the left and right lateral bending was not significant (18% and 12%, respectively). The improved fixation had minimal influence on the adjacent segments. CONCLUSIONS: Combined anterior posterior fixation further reduces the segmental motion by almost 50% in flexion and extension, 33% and 39% in torsion, and does not significantly alter intradiscal pressure and spinal motion at adjacent segments.

Intradiscal pressure and kinematic behavior of lumbar spine after bilateral laminotomy and laminectomy.

BACKGROUND CONTEXT: Bilateral laminotomy has been proposed as an alternative to laminectomy for decompression of lumbar spinal stenosis. Preservation of the posterior midline ligaments with laminotomy is presumed to maintain spinal segment stability. There have been no previous studies that directly compare the amount of destabilization and increase in disc pressures between the two procedures. PURPOSE: To quantify spinal segmental instability caused by bilateral laminotomy and laminectomy, and to compare the central and peripheral intradiscal pressures after the two procedures. STUDY DESIGN/SETTING: Mechanical testing of the lumbar motion segments of calf spines. METHODS: Nine fresh calf spines were tested under flexion, extension, lateral bending and axial rotation, intact first, then after laminotomy and laminectomy at the level of L4-L5. Four miniature pressure transducers were implanted in the central and peripheral disc at L4-L5 to measure intradiscal pressures. Three-dimensional motion was measured with motion analysis system. RESULTS: Comparing with bilateral laminotomy, laminectomy showed significant increase in segmental motion at the surgical level in flexion (16%, p<.05), extension (14%, p<.04) and right axial rotation (23%, p<.03). In flexion, the stress at the anterior annulus increased a nonsignificant 20% after laminotomy, but significant 130% after laminectomy (p<.02). In the intact spine, the posterolateral annulus experienced the highest stress with lateral bending to the same side when compared with other loading directions. This stress remained unchanged after laminotomy but increased 9% after laminectomy (p<.06). In rotation, axial intradiscal stresses were evenly distributed and unchanged after each procedure. CONCLUSIONS: Laminectomy causes more destabilization of a spinal motion segment than laminotomy and significantly increases disc stress in the anterior annulus.