Effects of Lumbosacral Arthrodesis on the Biomechanics of the Sacroiliac Joint.

BACKGROUND: It is unclear whether the sacroiliac joint is vulnerable to adjacent segment disease. Clinical studies have suggested that many patients who have undergone lumbar arthrodesis will develop adjacent segment disease, which may contribute to sacroiliac joint degeneration. The purpose of the present study was to examine whether arthrodesis in the lumbar spine results in altered biomechanics at the sacroiliac joint that could contribute to adjacent segment disease within the joint. METHODS: With use of human cadavers in a biomechanical laboratory study, the effects of lower-lumbar arthrodesis and sacroiliac screws on the biomechanics of the sacroiliac joint were assessed. Human cadaveric pelves with lumbar spines were biomechanically tested in flexion-extension, rotation about the vertical axis, and compression along the vertical axis with single and double-leg support. Four conditions were compared: (1) intact, (2) L4-L5 arthrodesis, (3) L4-S1 arthrodesis, and (4) left sacroiliac screw. Construct vertical and horizontal motions at the anterior and posterior surfaces of the sacroiliac joint were measured. RESULTS: Significant measurable increases in motion of the sacroiliac joint related to arthrodesis of the lumbar spine occurred with flexion-extension loading (p < 0.05). No significant changes were observed for rotation about the vertical axis or compression along the vertical axis with single and double-leg support. CONCLUSIONS: After 360°, 1 or 2-level lumbosacral spine arthrodesis, the sacroiliac joint showed a significant increase in rotational motion with flexion-extension loading. Increases in horizontal translation with axial rotation loading and vertical translation with axial compression loading were not significant. CLINICAL RELEVANCE: The risk of significant alteration of normal sacroiliac kinematics should be considered in all patients undergoing 360° lumbosacral arthrodesis.

Comparison of Biomechanical Properties of a Synthetic L3-S1 Spine Model and Cadaveric Human Samples.

Human cadavers currently represent the gold standard for spine biomechanical testing, but limitations such as costs, storage, handling, and high interspecimen variance motivate the development of alternatives. A commercially available synthetic surrogate for the human spine, the Sawbones spine model (SBSM), has been developed. The equivalence of SBSM to a human cadaver in terms of biomechanical behavior has not been fully assessed. The objective of this study is to compare the biomechanics of a lumbar tract of SBSM to that of a cadaver under physiologically relevant mechanical loads. An L3-S1 SBSM and 39 comparable human cadaver lumbar spine tracts were used. Each sample was loaded in pure flexion-extension or torsion. Gravity and follower loads were also included. The movement of each vertebral body was tracked via motion capture. The range of motion (ROM) of each spine segment was recorded, as well as the overall stiffness of each L3-S1 sample. The ROM of SBSM L3-L4 was larger than that found in cadavers in flexion-extension and torsion. For the other spine levels, the ROMs of SBSM were within one standard deviation from the mean values measured in cadavers. The values of structural stiffness for L3-S1 of SBSM were comparable to those of cadaveric specimens for both flexion and torsion. In extension, SBSM was more compliant than cadavers. In conclusion, most of the biomechanical properties of an L3-S1 SBSM model were comparable to those of human cadaveric specimens, supporting the use of this synthetic surrogate for testing applications.

Biomechanical Comparison between Volar Plate Fixator and Nonbridge External Wrist Fixator.

Intra-articular distal radius fractures are difficult to reduce and maintain by nonoperative means. ORIF leaves implants in the patient long after the fracture is healed. External fixation can stabilize the reduced fracture and leaves no long-term implants. The nonbridging fixator (NBX) will provide better reduction and comparable rigidity of fixation to a volar plate for a 5-fragment, OTA 23 C3.2 distal radius fracture. A 5-part distal radius fracture was created in 5 pairs of cadaver arms. One arm was randomly fixed with the NBX fixator; the matched pair was fixed with a volar plate (VPS). Fluoroscopic images recorded the extremes of passive volar-dorsiflexion range of motion (ROM) and radial-ulnar deviation ROM. Each arm was loaded with an axial force at a constant displacement rate until failure. The average reduction of radial tilt achieved for the NBX group was 13.8 ± 4.8° and 6.3 ± 4.7° for VPS; radial length: 3.4 ± 3.7 mm for NBX and 1.9 ± 1.0 mm for VPS; volar tilt: 26.3 ± 12.4° for NBX and 14.0 ± 13.5° for VPS. For NBX, ROM was slightly less after fixation than before fracture. ROM with volar plating was greater after fracture. The peak axial load for NBX was 925 ± 445 N; for VPS, 2,152 ± 1023 N. NBX had minimal effect on ROM and provided adequate strength and restoration of alignment at least as good as VPS for this 5-part fracture model.

Effects of gamma irradiation on the biomechanical properties of peroneus tendons.

PURPOSE: This study was designed to investigate the biomechanical properties of nonirradiated (NI) and irradiated (IR) peroneus tendons to determine if they would be suitable allografts, in regards to biomechanical properties, for anterior cruciate ligament reconstruction after a dose of 1.5-2.5 Mrad. METHODS: Seven pairs of peroneus longus (PL) and ten pairs of peroneus brevis (PB) tendons were procured from human cadavers. The diameter of each allograft was measured. The left side of each allograft was IR at 1.5-2.5 Mrad, whereas the right side was kept aseptic and NI. The allografts were thawed, kept wet with saline, and attached in a single-strand fashion to custom freeze grips using liquid nitrogen. A preload of 10 N was then applied and, after it had reached steady state, the allografts were pulled at 4 cm/sec. The parameters recorded were the displacement and force. RESULTS: The elongation at the peak load was 10.3±2.3 mm for the PB NI side and 13.5±3.3 mm for the PB IR side. The elongation at the peak load was 17.4±5.3 mm for the PL NI side and 16.3±2.0 mm for the PL IR side. For PL, the ultimate load was 2,091.6±148.7 N for NI and 2,122.8±380.0 N for IR. The ultimate load for the PB tendons was 1,485.7±209.3 N for NI and 1,318.4±296.9 N for the IR group. The ultimate stress calculations for PL were 90.3±11.3 MPa for NI and 94.8±21.0 MPa for IR. For the PB, the ultimate stress was 82.4±19.0 MPa for NI and 72.5±16.6 MPa for the IR group. The structural stiffness was 216.1±59.0 N/mm for the NI PL and 195.7±51.4 N/mm for the IR side. None of these measures were significantly different between the NI and IR groups. The structural stiffness was 232.1±45.7 N/mm for the NI PB and 161.9±74.0 N/mm for the IR side, and this was the only statistically significant difference found in this study (P=0.034). CONCLUSION: Our statistical comparisons found no significant differences in terms of elongation, ultimate load, or ultimate stress between IR and NI PB and PL tendons. Only the PB structural stiffness was affected by irradiation. Thus, sterilizing allografts at 1.5-2.5 Mrad of gamma irradiation does not cause major alterations in the tendons' biomechanical properties while still providing a suitable amount of sterilization for anterior cruciate ligament reconstruction.

Biomechanical analysis of four- versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures.

BACKGROUND CONTEXT: Conventionally, short-segment fusion involves instrumentation of one healthy vertebra above and below the injured vertebra, skipping the injured level. This short-segment construct places less surgical burden on the patient compared with long-segment constructs, but is less stable biomechanically, and thus has resulted in clinical failures. The addition of two screws placed in the fractured vertebral body represents an attempt to improve the construct stiffness without sacrificing the benefits of short-segment fusion. PURPOSE: To determine the biomechanical differences between four- and six-screw short-segment constructs for the operative management of an unstable L1 fracture. STUDY DESIGN: Biomechanical study of instrumentation in vertebral body cadaveric models simulating an L1 axial load injury pattern. METHODS: Thirteen intact spinal segments from T12 to L2 were prepared from fresh-frozen cadaver spines. An axial load fracture of at least 50% vertebral body height was produced at L1 and then instrumented with pedicle screws. Specimens were evaluated in terms of construct stiffness, motion, and rod strain. Two conditions were tested: a four-screw construct with no screws at the L1 fractured body (4S) and a six-screw construct with screws at all levels (6S). The two groups were compared statistically by paired Student t test. RESULTS: The mean stiffness in flexion-extension was increased 31% (p<.03) with the addition of the two pedicle screws in L1. Relative motion in terms of vertical and axial rotations was not significantly different between the two groups. The L1-L2 rod strain was significantly increased in the six-screw construct compared with the four-screw construct (p<.001). CONCLUSIONS: In a cadaveric L1 axial load fracture model, a six-screw construct with screws in the fractured level is more rigid than a four-screw construct that skips the injured vertebral body.

Comparison of 2 forearm reconstructions for longitudinal radioulnar dissociation: a cadaver study.

PURPOSE: We present the results of a cadaveric study of 2 forearm reconstructions with radial head replacement for longitudinal radioulnar dissociation injuries. METHODS: We created a simulated longitudinal radioulnar dissociation injury in 8 cadaver forearms. Two reconstructions were performed alternately on each arm: patellar tendon interosseous ligament complex reconstruction and the Herbert sling extensor retinaculum plication. We performed mechanical testing in a materials testing machine with and without a radial head replacement, and measured ulnocarpal impaction force through 2 distal ulna strain gauges. We determined relative radioulnar displacement using live fluoroscopic analysis of implanted stainless-steel beads. RESULTS: Relative radioulnar longitudinal displacement in the destabilized forearms was 10.7 compared with 0.7 mm before creating the injury. A prosthetic radial head replacement alone decreased the displacement by 75% to 2.7 mm. Interosseous ligament reconstruction alone reduced the displacement to 5.1 mm and to 1.3 mm when combined with a radial head implant. The Herbert sling alone did not improve longitudinal stability. The distal ulna force in the native arm was 17 N, or 17% of the force across the wrist. The interosseous ligament reconstruction restored the force to 21 N, whereas the Herbert sling only marginally decreased the ulna impaction force to 45 N. Adding a radial head decreased the distal ulna force to 7 N for the patellar tendon interosseous ligament reconstruction, and 2 N for the Herbert sling. CONCLUSIONS: In longitudinal radioulnar dissociation injuries, the radial head is an important stabilizer and should be repaired or replaced to minimize radial shortening and ulnar impaction force. Patellar tendon interosseous ligament reconstruction effectively restores the ulnocarpal force distribution and markedly reduces longitudinal instability at the distal radioulnar joint. Combined with radial head arthroplasty, the construct has stability similar to an intact forearm. The Herbert sling did not improve longitudinal stability in this testing construct. CLINICAL RELEVANCE: Treatment of longitudinal radioulnar dissociation may benefit from radial head replacement and interosseous ligament reconstruction using a patellar tendon graft.

Building a biomechanical model.

This article is based on an invited presentation at the Biomechanics Session for the Basic Science Focus Forum held at the Orthopaedic Trauma Association meeting, 2010. It is not intended to be a scientific presentation of any specific investigation. It presented aspects of several types of investigations to illustrate the variety of biomechanical models that are used, and what value can be derived from those models. All models have limitations in what they try to portray. In order for any model to provide useful information, it must stand some type of validation of its ability to behave similar to real world experiences. The advantages and disadvantages of each model must be described to the best of the investigators ability-so that the readers can determine how the information may be used in the real world.

The role of soft tissues in plate fixation of proximal phalanx fractures.

The tension band effect of plate fixation and the contribution of soft tissues to that effect was examined biomechanically in human proximal phalanges. Forty-six proximal phalanges in whole cadaver hands with all soft tissues in place (intact) and 43 proximal phalanges stripped of soft tissues (denuded) were tested. After midshaft osteotomy, each proximal phalanx was fixed internally with a dorsal minicondylar plate, a lateral minicondylar plate, a dorsal straight plate, or a lateral straight plate. Specimens were tested in three-point apex dorsal bending to clinical failure, defined as 30 degrees angulation. Ultimate moment (stability) at this angulation was similar among the four fixation methods in the specimens with all soft tissues intact. Stability also was similar among these methods in the denuded specimens. There were no significant differences in stability between minicondylar and straight plates or between dorsal and lateral plates in the specimens with soft tissues, nor were there significant differences between these groups in the denuded specimens. The stability of the four fixation methods was significantly greater in the specimens with soft tissues than in the denuded specimens. Soft tissues increased the stability of lateral minicondylar plates by 163%, lateral straight plates by 157%, dorsal minicondylar plates by 126%, and dorsal straight plates by 104%, providing a dorsal tension band effect that counteracted the buttress (compression) of the volar fracture surfaces of the phalanx. The results suggest that in the clinical setting a laterally placed straight or minicondylar plate may provide as much stability to a phalanx with a midshaft fracture as does the traditional, more invasive dorsally placed minicondylar or straight plate. These findings must be evaluated with caution, however, because all specimens were from embalmed cadavers, and the formalin fixation may have augmented the stability and stiffness of the soft tissues in the intact specimens. A subsequent pilot study comparing intact proximal phalangeal specimens that were formalin-fixed with those that were fresh-frozen showed a significant increase in stability and stiffness of formalin-fixed specimens.

Role of soft tissues in metacarpal fracture fixation.

The contribution of soft tissues in stabilizing fracture fixation in metacarpals is appreciated clinically, but no quantitative biomechanical study of their role has been done. All previous studies of fracture fixation in vitro have been done on metacarpals denuded of soft tissues. To quantify the role of soft tissues in metacarpal fracture fixation, the biomechanical effectiveness of four fixation devices was examined in human cadaver metacarpals with and without soft tissues. Values were compared for three nonrigid methods (expandable intramedullary fixation devices, crossed Kirschner wires, and single half-pin frames) and one rigid method (dorsal plates) in 45 disarticulated metacarpals stripped of soft tissues (denuded) and in 46 metacarpals in whole hands with all soft tissues remaining (intact). Mechanical testing to complete failure in three-point apex dorsal bending was done in all specimens. Ultimate moment (strength) of each of the four fixation methods was significantly greater in intact specimens than in denuded specimens. Crossed Kirschner wires were most stable in intact specimens, and dorsal plates were more stable in denuded specimens. The results show that soft tissues contribute to the strength of fracture fixation. Clinically, surgeons may be able to use a less invasive fixation method than plating without compromising the strength of metacarpal fixation in patients whose soft tissues are not severely disrupted and the fracture configuration allows. Plating may offer optimum stability in patients whose soft tissues are damaged severely and provide less strengthening of the fracture construct.