Clinical performance of an elastomeric lumbar disc replacement: Minimum 12 months follow-up.

BACKGROUND: Elastomeric disc replacements have been developed to restore normal shock absorption and physiologic centers of rotation to the degenerated disc. The Physio-L Artificial Lumbar Disc is an elastomeric disc which uses a compliant polycarbonate-polyurethane core with enhanced endurance properties. The objective of this study was to evaluate the safety and efficacy of the Physio-L through a 12-month follow-up period in a prospective, nonrandomized clinical trial. METHODS: Twelve patients who met the inclusion/exclusion criteria were enrolled in the study. Eight patients received a single implant (L5-S1) and 4 received a 2-level implantation (L4-5 and L5-S1). Patients were assessed preoperatively and postoperatively at 6 weeks and 3, 6, and 12 months. Primary outcomes included the VAS, ODI, a radiographic analysis of implant condition, incidence of major complications, and reoperations. Secondary outcomes included SF-36, ROM at index and adjacent levels and disc height. RESULTS: All patients completed the 12-month follow-up evaluations. Through 12 months, the Physio-L devices have remained intact with no evidence of subsidence, migration, or expulsion. VAS low-back pain and ODI scores improved significantly at all follow-up periods compared to preoperative scores. The range of motion of 13.3° ± 5.5° at the index level was considered normal. Overall, patients were satisfied with an average score of 83.5 ± 26.8 mm. When comparing the device to other artificial discs, the current device showed a clinically relevant improvement in both ODI and VAS scores at all follow-up time points. Statistically significant improvements in both scores were observed at 12 months (P < .05). CONCLUSION: The Physio-L is safe and efficacious, as demonstrated by improved pain relief and functional recovery without any implant failures, significant device related complications, or adverse incidents. The clinical results for VAS and ODI were superior to other marketed artificial lumbar discs such as the Charité and ProDisc-L at the same follow-up timeframes.

Load balance in total knee arthroplasty: an in vitro analysis.

BACKGROUND: One of the goals of total knee arthroplasty (TKA) is to balance the loads between the compartments of the knee. An instrumented load cell that measures compartment loads in real time is utilized to evaluate conventional, qualitative methods of achieving this balance. METHODS: TKA was performed on 10 cadaveric knees. Prior to and after load balancing, compartment forces were measured at flexion angles of 0-90 degrees. Knees were randomly assigned into one of two groups, based upon whether or not the surgeons could visualize the load cell's output during balancing. RESULTS: Prior to attempting load balance, there were significant differences between the medial and lateral compartment loads for all knees (p < 0.05). After attempting balance with the aid of the load cell, there was equal load balance at all angles studied. Without the aid of the load cell, balance was not consistently achieved at every angle. CONCLUSIONS: Conventional load balancing techniques in TKA are not perfect.

Biomechanical comparison of intramedullary nail and blade plate fixation for tibiotalocalcaneal arthrodesis.

OBJECTIVE: Tibiotalocalcaneal arthrodesis is used to manage severe bone loss, arthritis, and/or instability. The goal is to relieve pain through a stable, well-aligned hindfoot and ankle. The purpose of this study was 2-fold: to biomechanically compare 1) initial stability, and 2) the effect of bone density on the stability of intramedullary nail and blade plate fixation in tibiotalocalcaneal arthrodesis. DESIGN: Biomechanical study using anatomic specimens. METHODS: Ankle and subtalar joint capsules were exposed for 7 pairs of fresh-frozen anatomic specimens. One ankle from each pair was instrumented with an interlocked intramedullary nail inserted retrograde across the subtalar and ankle joint while the contralateral hind foot was stabilized with a lateral cannulated blade plate. Specimen stability was tested in plantar/dorsiflexion and inversion/eversion to a maximum bending moment of 12 Nm and in internal/external rotation to a maximum torque of 7 Nm. Physical measurements of bone density were made to determine its effect on stability. MAIN OUTCOME MEASURES: Maximum angular displacement of the constructs in plantarflexion, dorsiflexion, inversion, eversion, internal rotation, and external rotation. RESULTS: No significant differences were observed between the plated and nailed constructs in the 3 loading configurations (Power = 0.77). Only 6 pairs were included in the results because of fixation failures. A small but significant reduction in internal rotation alone of 1.8 degrees was found with the plated compared with the nailed construct (P = 0.045). Reduced stability was associated with lower bone density in torsion and inversion/eversion in the plated constructs (r = 0.67- 0.87) with a similar trend seen in torsion in the nailed constructs (r = 0.5). CONCLUSION: Initial construct stabilities and the effect of reduced bone density were found to be similar between the blade plate and the intramedullary nail in tibiotalocalcaneal arthrodesis, thus implant choice may be based on other clinical factors, such as surgeon preference or soft-tissue status.

Biomechanical stability of intramedullary nailed high proximal third tibial fractures with cement augmented proximal screws.

OBJECTIVE: Intramedullary nailing of nonarticular proximal tibia fractures can be affected by bone density resulting in loss of stability, fixation, and malalignment in osteopenic bone. This study was designed to quantify the biomechanical effects of augmenting proximal screws with cement in intramedullary nailing of high proximal third tibial fractures. DESIGN: In vitro biomechanical study using anatomic specimens. METHODS: Reamed nails were inserted into seven pairs of fresh-frozen cadaveric proximal tibiae and secured using two oblique and two transverse proximal screws. Paired tibiae were randomly assigned into two groups: cemented and noncemented proximal screw-holes. Bone cement was injected into the screw-holes before screw insertion in the cemented tibiae. Specimens were then tested in flexion/extension and varus/valgus to 12 Nm and in torsion to 7 Nm. Physical measurements of bone density were obtained to determine the effect of density on stability. MAIN OUTCOME MEASURES: Stability of the construct in both groups was analyzed and compared statistically using paired t tests. RESULTS: Cement augmentation of the proximal screws significantly increased mechanical stability in torsion and varus/valgus load configurations, with average decreases in rotational motion of 5.4 degrees +/- 1.6 degrees and 5.1 degrees +/- 5 degrees respectively. No change in stability was observed in flexion/extension loading. A trend toward decreased stability was seen in the uncemented construct in varus/valgus; cement augmentation of the proximal screws eliminated this effect. CONCLUSIONS: Lower bone density decreased the stability of the uncemented construct; however, cement augmentation of the proximal screws showed a trend to eliminate this effect in the varus/valgus loading configuration and should be considered when nailing proximal third tibial fractures in osteoporotic patients.

Biomechanical assessment of stability in the metastatic spine following percutaneous vertebroplasty: effects of cement distribution patterns and volume.

Percutaneous vertebroplasty is a minimally invasive, radiologically guided procedure whereby bone cement is injected into structurally weakened vertebrae to provide added biomechanical stability. In addition to treating osteoporotic vertebral fractures, this technique is also used to relieve pain by stabilizing metastatically compromised vertebrae that are at risk of pathologic burst fracture. Optimal cement distribution patterns to improve biomechanical stability to metastatically involved vertebral bodies remain unknown. This study aimed to determine the effect of cement location and volume of cement injected during percutaneous vertebroplasty on improving vertebral stability in a metastatically-compromised spinal motion segment using a parametric poroelastic finite element model. A three-dimensional parametric finite element model of a thoracic spinal motion segment was developed and analyzed using commercially available software. A total of 16 metastatic pre and post vertebroplasty scenarios were investigated using a serrated spherical representation of tumor tissue and various geometric representations of polymethylmethacrylate (PMMA). The effect of vertebroplasty on vertebral bulge, a measure of posterior vertebral body wall motion as an indicator of burst fracture initiation, was assessed. In all cases, vertebroplasty reduced vertebral bulge, but the risk of the initiation of burst fracture was minimized with cement located posterior to the tumor, near the posterior vertebral body wall. Vertebral bulge decreased by up to 62% with 20% cement injection. These findings demonstrate that location and distribution of cement within the vertebral body has a noticeable effect on the restoration of biomechanical stability following percutaneous vertebroplasty.

Pressurization of the metastatic femur during prophylactic intramedullary nail fixation.

BACKGROUND: Venting during prophylactic nailing of metastatic femoral lesions may decrease intramedullary pressures and the risk of fat and tumor embolization. This study aimed to quantify maximum intramedullary pressure levels and durations, and to examine tumor displacement during prophylactic reamed intramedullary nailing of vented and nonvented metastatically involved femora. METHODS: Proximal metastatic defects were simulated in six pairs of cadaveric femora and prophylactically treated using reamed intramedullary nailing with or without proximal and distal vents. Maximum pressures, durations of pressure exceeding embolic threshold levels (25 and 200 mm Hg), and tumor displacement were recorded. RESULTS: Maximum pressures and durations of pressure exceeding embolic thresholds decreased more than 50% with venting (p < 0.05). Distal pressures surpassed the 25-mm Hg threshold even with venting. Post-test sectioning showed increased tumor disbursement in vented specimens. CONCLUSIONS: Venting reduces pressurization during prophylactic reamed intramedullary nailing of femoral metastases, but may increase the spread of tumor to extraskeletal tissue if vented tissue is not contained.

Metastatic burst fracture risk prediction using biomechanically based equations.

Clinical guidelines are a useful adjunct to select patients with spinal metastases for prophylactic intervention. The objective of this study is to determine the ability of biomechanically based models to accurately predict metastatic burst fracture risk. Ninety-two vertebrae with osteolytic spinal metastases were examined retrospectively. Vertebrae were categorized as burst fractured, wedge fractured, or intact and analyzed using three predictive models: vertebral bulge (maximum radial displacement under load), vertebral axial displacement (maximum axial displacement under load), and a volumetric estimate of tumor size. The load-bearing capacity parameter (tumor volume, bone mineral density, disc quality, pedicle involvement) was determined from computed tomography while the load-bearing requirement parameter (pressure load, loading rate) was determined using computed tomography and patient records (retrieved for 37 patients [52%]). Fracture prediction was optimized using the vertebral bulge model considering only load-bearing capacity with a specificity, sensitivity, and confidence interval of 1 to yield a clear threshold for burst fracture risk. Fracture prediction in the other two models, vertebral axial displacement considering only load-bearing capacity and tumor size, also was strong with receiver-operator curve values of 0.992 and 0.988, respectively. The predictive power of these models can provide useful clinical information for prophylactic decision-making.

An anthropometric study of the distal ulna: implications for implant design.

Arthroplasty of the distal ulna is frequently used for distal radioulnar joint disorders; however, prosthetic failures are common. An implant replicating the ulna may be advantageous from the viewpoint of joint kinematics, wear, and fixation. The geometry of 40 distal ulnas was quantified by computed tomography scanning and radiography. Ulnar head diameter (16.8 +/- 1.6 mm), canal diameter (8.3 +/- 1.6 mm), eccentricity of the head relative to the shaft (2.5 +/- 1.4 mm), and angle of articulation (176 degrees +/- 29 degrees ) were measured. No correlation was found between the diameters of the ulnar head and intramedullary canal (R(2) = 0.2). A modular head-stem system with an eccentric offset may best reproduce the geometry of the distal ulna.