Increasing loads and diminishing returns: a biomechanical study of direct vertebral rotation.

STUDY DESIGN: Biomechanical simulation of DVR and pure-moment testing on thoracic spines. OBJECTIVES: Characterize load-deformation response of thoracic spines under DVR maneuvers until failure, and compare to pure-moment testing of same spines. Despite reports of surgical complications, few studies exist on increase in ROM under DVR torque. Biomechanical models predicting increases from surgical releases have consistently used "pure-moments", a standard established for non-destructive measurement of ROM. Yet, DVR torque is not accurately modeled using pure moments and, moreover, magnitudes of torque applied during DVR maneuvers may be substantially higher than pure-moment testing. METHODS: Cadaveric thoracic spines (N = 11) were imaged, then prepared. Polyaxial pedicle screws were implanted at T7-T10 after surgical releases. Bilateral facetectomies and Ponte osteotomies were completed at T10-T11. A custom apparatus, mounted into an 8-dof MTS load frame, was used to attach to pedicle screws, allowing simulation of surgical DVR maneuvers. Motions of vertebrae were measured using optical motion tracking. Torque was increased until rupture of the T10-T11 disc or fracture at the pedicle screw sites at any level. The torque-rotation behavior was compared to its behavior under pure-moment testing performed prior to the DVR maneuver. RESULTS: Under DVR maneuvers, failure of the T10-T11 discs accompanied in most cases by pedicle screw loosening, occurred at 13.7-54.7 Nm torque, increasing axial rotation by 1.4°-8.9°. In contrast, pure-moment testing (4 Nm) increased axial rotation by only 0.0°-0.9°. CONCLUSIONS: DVR resulted in substantially greater correction potential increases compared to pure-moment testing even at the same torque. These results suggest increased flexibility obtained by osteotomies and facetectomies is underestimated using pure-moment testing, misrepresenting clinical expectations. The present study is an important and necessary step toward the establishment of a more accurate and ultimately surgically applied model. LEVEL OF EVIDENCE: III.

Biomechanical Comparison of Fixation Devices for First Metatarsocuneiform Joint Arthrodesis.

Common surgical treatment of first tarsal-metatarsal arthritis is by first metatarsocuneiform joint arthrodesis. While crossed-screw and locking plate fixation are the most widely used methods, a novel construct was designed to alleviate soft tissue irritation while still providing stable fixation. Using anatomic first metatarsal and medial cuneiform composites, we compared 3 arthrodesis implants (crossed-screw, dorsal locking plate, and IO Fix) under 2 cyclic bending loading scenarios (cantilever and 4-point bending). Additionally, the optimal orientation (plantar-dorsal or dorsal-plantar) of the IO Fix construct was determined. Failure load, diastasis, joint space angle, and axial and angular stiffness were determined. Both crossed-screw fixation and the IO Fix constructs experienced significantly higher failure loads than the dorsal locking plate during both loading scenarios. Additionally, they had lower plantar diastasis and joint space angle at failure than the plate. Moreover, the plantar-dorsal IO Fix construct was significantly stiffer than the crossed-screw during cantilever bending. Finally, the plantar-dorsal orientation of the IO Fix device had higher failure load and lower diastasis and angle at failure than in the dorsal-plantar orientation. The results suggest that the IO Fix system can reduce motion at the interfragmentary site and ensure compression for healing comparable to that of the crossed-screw fixation. LEVELS OF EVIDENCE: Level V: Bench testing.

Periprosthetic femoral bone loss in total hip arthroplasty: systematic analysis of the effect of stem design.

INTRODUCTION: Periprosthetic bone loss may lead to major complications in total hip arthroplasty (THA), including loosening, migration, and even fracture. This study analysed the influence of femoral implant designs on periprosthetic bone mineral density (BMD) after THA. METHODS: The results of all previous published studies reporting periprosthetic femoral BMD following THA were compiled. Using these results, we compared percent changes in bone loss as a function of: femoral stem fixation, material, and geometry. RESULTS: The greatest bone loss was in the calcar region (Gruen Zone 7). Overall, cemented stems had more bone loss distally than noncemented stems, while noncemented stems had more proximal bone loss than cemented stems. Within noncemented stems, cobalt-chromium (CoCr) stems had nearly double the proximal bone loss compared to titanium (Ti) alloy stems. Finally, within noncemented titanium alloy group, straight stems had less bone loss than anatomical, tapered, and press-fit designs. DISCUSSION: The findings from the present study quantified percent changes in periprosthetic BMD as a function of fixation method, alloy, and stem design. While no one stem type was identified as ideal, we now have a clearer understanding of the influence of stem design on load transfer to the surrounding bone.

Distal fibula fracture fixation: Biomechanical evaluation of three different fixation implants.

BACKGROUND: The goal of this study was to evaluate the biomechanical performance of three distal fibula fracture fixation implants in a matched pair cadaveric fibula model: (1) a 5-hole compression plate with lag screw, (2) a 5-hole locking plate with lag screw, and (3) the 6-hole tabbed-plate with locking screws. METHODS: Three-dimensional motions between the proximal and distal fibular segments were measured under cyclic valgus bending, cyclic compressive axial loading, and cyclic torsional external-rotation loading. During loading, strains were measured on the surfaces of each fibula near the simulated fracture site, and on the plate, to assess load transfer. Bone quality was quantified globally for each donor using bone mineral density (BMD) measured using Dual X-ray absorptiometry (DEXA) and locally at the fracture site using bone mineral content (BMC) measured using peripheral quantitative computed tomography (pQCT). RESULTS: Mean failure loads were below 0.2Nm of valgus bending and below 4Nm of external-rotational torque. Mean failure angulation was below 1degree for valgus bending, and failure rotation was below 7degrees for external-rotation. In the compression plate group, significant correlations were observed between bone quality (global BMD and local BMC) and strain in every one of the five locations (Pearson correlation coefficients >0.95, p<0.05). In contrast, in the locking and tabbed-plate groups, BMD and BMC correlated with far fewer strain locations. CONCLUSIONS: Overall, the tabbed-plate had similar construct stability and strength to the compression and locking plates. However, the distribution of load with the locking and tabbed-plates was not as heavily dependent on bone quality.

Static and dynamic fatigue behavior of topology designed and conventional 3D printed bioresorbable PCL cervical interbody fusion devices.

Recently, as an alternative to metal spinal fusion cages, 3D printed bioresorbable materials have been explored; however, the static and fatigue properties of these novel cages are not well known. Unfortunately, current ASTM testing standards used to determine these properties were designed prior to the advent of bioresorbable materials for cages. Therefore, the applicability of these standards for bioresorbable materials is unknown. In this study, an image-based topology and a conventional 3D printed bioresorbable poly(ε)-caprolactone (PCL) cervical cage design were tested in compression, compression-shear, and torsion, to establish their static and fatigue properties. Difficulties were in fact identified in establishing failure criteria and in particular determining compressive failure load. Given these limitations, under static loads, both designs withstood loads of over 650 N in compression, 395 N in compression-shear, and 0.25 Nm in torsion, prior to yielding. Under dynamic testing, both designs withstood 5 million (5M) cycles of compression at 125% of their respective yield forces. Geometry significantly affected both the static and fatigue properties of the cages. The measured compressive yield loads fall within the reported physiological ranges; consequently, these PCL bioresorbable cages would likely require supplemental fixation. Most importantly, supplemental testing methods may be necessary beyond the current ASTM standards, to provide more accurate and reliable results, ultimately improving preclinical evaluation of these devices.

Accuracy of radiographs in assessment of displacement in lateral humeral condyle fractures.

PURPOSE: Determining the magnitude of displacement in pediatric lateral humeral condyle fractures can be difficult. The purpose of this study was to (1) assess the effect of forearm rotation on true fracture displacement using a cadaver model and to (2) determine the accuracy of radiographic measurements of the fracture gap. METHODS: A non-displaced fracture was created in three human cadaveric arms. The specimens were mounted on a custom apparatus allowing forearm rotation with the humerus fixed. First, the effect of pure rotation on fracture displacement was simulated by rotating the forearm from supination to pronation about the central axis of the forearm, to isolate the effects of muscle pull. Then, the clinical condition of obtaining a lateral oblique radiograph was simulated by rotating the forearm about the medial aspect of the forearm. Fracture displacements were measured using a motion-capture system (true-displacement) and clinical radiographs (apparent-displacement). RESULTS: During pure rotation of the forearm, there were no significant differences in fracture displacement between supination and pronation, with changes in displacement of <1.0 mm. During rotation about the medial aspect of the forearm, there was a significant difference in true displacements between supination and pronation at the posterior edge (p < 0.05). CONCLUSION: Overall, true fracture displacement measurements were larger than apparent radiographic displacement measurements, with differences from 1.6 to 6.0 mm, suggesting that the current clinical methods may not be sensitive enough to detect a displacement of 2.0 mm, especially when positioning the upper extremity for an internal oblique lateral radiograph.

Biomechanical comparison of syndesmotic injury fixation methods using a cadaveric model.

BACKGROUND: There is growing interest in suture-button devices for syndesmosis injury, which are intended to offer less rigid fixation than screw fixation. METHODS: The fixation strength with 2 different suture-button devices, ZipTight and TightRope, were compared using 5 cadaveric leg pairs (n = 10). In an additional 5 pairs (n = 10), ZipTight was compared to 3.5 mm quadricortical screw fixation. Ankle motion was measured intact, then following simulated syndesmosis injury and fixation. Cyclic loads (peak 750 N, 7.5 Nm) were applied. Finally, external rotation to failure was measured and failure mode was documented. RESULTS: Range of motion increased after simulated injury and fixation with all devices (max 14.5 degrees). In all groups, diastasis remained below 1.0 mm intact and below 2.0 mm during cyclic loading. Compared to intact, under load to failure, diastasis with ZipTight devices increased by 4.7 ± 1.3 mm and 7.6 ± 4.3 mm, with TightRope, 6.3 mm, and screw construct, 1.3 mm. ZipTight specimens rotated approximately 80 ± 22 degrees before failure, TightRope, 67 ± 13 degrees, screw constructs, 76 ± 27 degrees. Mean failure torque was between 22.2 ± 6.9 Nm and 28.1 ± 12.7 Nm for ZipTight, compared to 32.9 ± 8.0 Nm for TightRope (P = .07), and 30.1 ± 9.6 Nm for screw constructs (P = .03). The majority of suture-button constructs failed by fibular fracture (ZipTight = 6, TightRope = 4), the remaining by device pull-through (ZipTight = 3, TightRope = 1) and loosening (ZipTight = 1). Conversely, 3 of screw-fixed specimens failed by device failure, 2 from bone fracture. CONCLUSION: Suture-button devices provided torsional strength below that of screw fixation. However, all devices may provide failure torques well above 20 Nm, exceeding likely torques applied in casts during healing.(1,2,4) CLINICAL RELEVANCE: Suture-button devices appear to have provided adequate fixation strength for syndesmosis injuries.

Fixation of non-cemented total hip arthroplasty femoral components in a simulated proximal bone defect model.

An accelerated sequential proximal femoral bone loss model was used to measure the initial stability of three noncemented femoral stem designs: fully porous-coated, proximally porous-coated, and dual-tapered, diaphyseal press-fit (N=18). Only dual-tapered, diaphyseal press-fit stems remained stable with as much as 105 mm of bone loss, with average cyclic micromotion remaining below 25 µm in ML and below 10 µm in AP planes. In contrast, with proximally coated and fully coated stem designs with circular or oval cross-sections, 60mm of bone loss, resulting in lower than 10 cm of diaphyseal bone contact length, led to gross instability, increasing average cyclic micromotions to greater than 100 µm prior to failure. Therefore, the results provide support for using a dual-tapered stem in revision cases with proximal bone loss.

Biomechanical testing of pin configurations in supracondylar humeral fractures: the effect of medial column comminution.

OBJECTIVES: We measured biomechanical stability in simulated supracondylar humeral fractures fixed with each of 6 pin configurations, 2 with associated medial comminution, and developed a technique for reproducible pin placement and divergence. METHODS: A transverse supracondylar osteotomy was performed on 36 biomechanical humerus models. Of these, 24 (4 groups of 6 specimens each) were fixed with pins in 1 of 4 lateral entry configurations. The remaining 12 (2 groups of 6 specimens each) had a 30-degree medial wedge removed from the distal humerus and were fixed with 1 of 2 configurations. Half of each group was tested under axial rotation and the other half under varus bending. The distal humerus was divided into 4 equal regions from lateral to medial (1-4). Lateral entry pins were inserted through regions 1-3, whereas the medial pin was inserted through region 4. RESULTS: Without comminution, 3 widely spaced, divergent lateral entry pins resulted in higher torsional stiffness (0.36 Nm/degree) than 2 pins in adjacent regions (P < 0.055), but similar to 2 pins in nonadjacent regions (P = 0.57). Three lateral entry pins had higher bending stiffness (79.6 N/mm) than 2 pins, which ranged from 46.7 N/mm (P < 0.01) to 62.5 N/mm (P = 0.21). With comminution, adding a third medial entry pin increased torsional stiffness (0.13-0.24 Nm/degree, P < 0.01) and increased bending stiffness (38.7-44.7 N/mm, P = 0.10). CONCLUSIONS: For fractures without medial column comminution, fixation using 3 lateral entry pins may provide the greatest combination of torsional and bending stiffness. With medial comminution, adding a third medial pin increased torsional stiffness (P < 0.01) and bending stiffness (P = 0.10).