Object. The goal of this study was to evaluate the comparative efficacy of three commonly used anterior thoracolumbar implants: the anterior thoracolumbar locking plate (ATLP), the smooth-rod Kaneda (SRK), and the Z-plate. Methods. In vitro testing was performed using the T9—L3 segments of human cadaver spines. An L-1 corpectomy was performed, and stabilization was achieved using one of three anterior devices: the ATLP in nine spines, the SRK in 10, and the Z-plate in 10. Specimens were load tested with 1.5-, 3-, 4.5-, and 6-Nm in flexion and extension, right and left lateral bending, and right and left axial rotation. Angular motion was monitored using two video cameras that tracked light-emitting diodes attached to the vertebral bodies. Testing was performed in the intact state in spines stabilized with one of the three aforementioned devices after the devices had been fatigued to 5000 cycles at ± 3 Nm and after bilateral facetectomy. There was no difference in the stability of the intact spines with use of the three devices. There were no differences between the SRK- and Z-plate—instrumented spines in any state. In extension testing, the mean angular rotation (± standard deviation) of spines instrumented with the SRK (4.7 ± 3.2°) and Z-plate devices (3.3 ± 2.3°) was more rigid than that observed in the ATLP-stabilized spines (9 ± 4.8°). In flexion testing after induction of fatigue, however, only the SRK (4.2 ± 3.2°) was stiffer than the ATLP (8.9 ± 4.9°). Also, in extension postfatigue, only the SRK (2.4 ± 3.4°) provided more rigid fixation than the ATLP (6.4 ± 2.9°). All three devices were equally unstable after bilateral facetectomy. The SRK and Z-plate anterior thoracolumbar implants were both more rigid than the ATLP, and of the former two the SRK was stiffer. Conclusions. The authors' results suggest that in cases in which profile and ease of application are not of paramount importance, the SRK has an advantage over the other two tested implants in achieving rigid fixation immediately postoperatively.

Object. Polymethylmethacrylate (PMMA) has long been used in the stabilization and reconstruction of traumatic and pathological fractures of the spine. Recently, hydroxyapatite (HA), an osteoconductive, biocompatible cement, has been used as an alternative to PMMA. In this study the authors compare the stabilizing effects of the HA product, BoneSource, with PMMA in an experimental compression fracture of L-1. Methods. Twenty T9—L3 cadaveric spine specimens were mounted individually on a testing frame. Light-emitting diodes were placed on the neural arches as well as the base. Motion was tracked by two video cameras in response to applied loads of 0 to 6 Nm. The weight-drop technique was used to induce a reproducible compression fracture of T-11 after partially coring out the vertebra. Load testing was performed on the intact spine, postfracture, after unilateral transpedicular vertebroplasty with 7 to 10 ml of PMMA or HA, and after flexion—extension fatiguing to 5000 cycles at ± 3 Nm. No significant difference between the HA- and PMMA cemented—fixated spines was demonstrated in flexion, extension, left lateral bending, or right and left axial rotation. The only difference between the two cements was encountered before and after fatiguing in right lateral bending (p ≤ 0.05). Conclusions. The results of this study suggest that the same angular rigidity can be achieved using either HA or PMMA. This is of particular interest because HA is osteoconductive, undergoes remodeling, and is not exothermic.

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Gastrocnemius contracture may be a significant cause of many foot disorders. Gastrocnemius tension can be estimated clinically by measuring maximum ankle dorsiflexion during full knee extension. Such measurements, when made with currently available goniometric devices, are subject to high levels of intra- and inter-observer variability. We have designed a device to more consistently measure ankle dorsiflexion, using three dimensional tracking sensors on the leg and foot. The applied dorsiflexion torque is kept constant by a computer, and the computer also monitors hindfoot position to maintain a neutrally aligned foot during testing. Repeated measurements on 26 feet were taken to determine the consistency of the device. The correlation coefficient for the measurements was 0.96, indicating very low intra-observer variability. The standard deviation of the repeated measures was 2 degrees. Based on the 95% confidence interval, the device can be considered accurate to within 4 degrees. Given this accuracy, this instrument could be used to assess gastrocnemius tension, its role in foot pathology, and the effectiveness of surgical lengthening. Compared to other currently available measuring devices, this instrument is the most reliable in estimating ankle dorsiflexion, since it is capable of controlling hindfoot position and applied dorsiflexion torque, and it can be easily constructed by other laboratories.

These results demonstrate few differences between angular rotation after either anterior or lateral TFC implantation. These findings add to data that find few differences between orientation of implanted TFCs. Combined with a decreased risk of adjacent structure injury through a lateral approach, these data support a lateral approach for lumbar interbody fusion.