Dwight G. Bronson

Experimental Tibial Plateau Fractures Augmented With Calcium Phosphate Cement Or Autologous Bone Graft

Background: Depression fractures of the tibial plateau are often managed with use of internal fixation and autologous bone-grafting to maintain an anatomical reduction. Bone-grafting, however, provides only limited stability. As calcium phosphate cements have appropriate mechanical properties, they may provide a more suitable alternative. The objective of this study was to compare the effect of a calcium phosphate cement with that of impacted cancellous autograft for maintaining an anatomical reduction in an experimental model of a tibial plateau fracture.Methods: Standardized cylindrical subchondral defects that were 8 mm in diameter and 10 mm deep were created bilaterally beneath the subchondral bone of the articular cartilage in the lateral tibial plateau of goats. An osteotome was used to fracture the overlying subchondral plate and articular cartilage. The plateau fracture fragment was completely depressed into the subchondral defect and then was anatomically reduced. The defects were randomly filled with either calcium phosphate cement or cancellous autograft. No internal fixation was used. The tibiae were harvested at varying time-periods that ranged from twenty-four hours to eighteen months. The stiffness of the healing augmented plateau fractures was determined. Histological specimens were assigned a score for degenerative changes. Loss of anatomic reduction was demonstrated in photomicrographs, and the amount of subsidence of the osteochondral fragment was measured in whole-mount histological sections.Results: The prevalence and degree of fracture subsidence was significantly reduced at all time-points in the defects treated with calcium phosphate cement compared with those filled with autograft (p < 0.05). There were no significant differences in fracture stiffness between the two treatment groups at any of the time-points examined. The calcium phosphate cement was rapidly resorbed, and the volume fraction of the calcium phosphate cement was decreased to 4% at six months. The trabecular bone volume in the defects was restored to that of the intact controls at six months in both treatment groups.Conclusions: Cancellous autograft did not maintain an anatomical reduction of the tibial plateau fractures in this model. In contrast, augmentation with calcium phosphate cement prevented subsidence of the fracture fragment and maintained articular congruency as the fracture healed. The improved articular congruency reduced the prevalence and severity of degenerative changes in the joint.Clinical Relevance: Reconstruction of intra-articular depression fractures of the tibial plateau is hampered by the loss of subchondral bone and the small fragment size. The use of calcium phosphate cement as a buttress in articular depression fractures may reduce the need for extensive internal fixation and allow earlier functional rehabilitation.

Stability of external circular fixation: a multi-variable biomechanical analysis

OBJECTIVE: To determine how the manipulation of the parameters of fixation and components of the circular external frame could improve and maintain optimal stability of bone fragments. DESIGN: We performed a multi-parametric biomechanical analysis of the extrinsic parameters effecting bone fragment stabilization. Results of testing are presented as a percent change in stiffness due to the manipulation of frame components and their interaction with other fixation parameters. BACKGROUND: Although there have been investigations of the biomechanical characteristics of circular external fixation, they have been limited to either individual frame components or full frame comparisons. Therefore, these studies did not provide a comprehensive understanding of how the manipulation of circular fixator components influences bone fragment stability. METHODS: Mechanical testing was performed in three phases examining the effect of numerous components including ring diameter, wire angle, ring separation, etc. on axial, torsional and bending stiffness. RESULTS: For phase I (single ring) and phase II (double-ring block), ring diameter was the most significant factor affecting axial and torsional stiffness, while wire angle, ring separation, and their interaction had the most influence on bending stiffness. Phase III (two double-ring blocks) showed that ring positioning with respect to the osteotomy site had the most affect on bending and torsional stiffness while axial stiffness was non-linear and dependent upon the applied load. CONCLUSIONS: The stability of bone fragments within a circular external fixator is affected by manipulation of the parameters of fixation or individual components of the frame. The contribution of each component to overall bone fragment stability is dependent upon the mode of loading. The changes in overall stability of bone fragments are dependent not only on the individual frame components but also upon their interaction with other parameters of fixation. RELEVANCE: Understanding how the manipulation of individual frame components will affect overall bone fragment stabilization will allow the surgeon to better control the stability of bone fragments for each clinical situation.

A Biomechanical Comparison of Open and Thoracoscopic Anterior Spinal Release in a Goat Model

Study Design. A biomechanical assessment of anterior release and discectomy in the thoracic spine was performed on an animal model using thoracoscopic and open thoracotomy techniques. Objectives. To compare the relative efficacy of these two techniques of release in achieving increased spinal mobility. Background Data. The clinical use of video‐assisted thoracoscopy in the correction of spinal deformity is increasing. The effectiveness of thoracoscopic anterior spinal release with discectomy has not been evaluated biomechanically. Methods. Anterior release with discectomy was performed on six midthoracic motion segments in five mature goats. The thoracoscopic technique was used for three levels on one side, and an open thoracotomy was used for the alternating three levels of the contralateral side. The duration of surgery for disc excision and the amount of blood loss for each technique were recorded. The intact cranial and caudal motion segments served as controls. The motion segments were individually subjected to nondestructive biomechanical testing. Torsional, sagittal, and coronal bending torques were applied, and the resulting angular displacement was measured. Results. The duration of surgery to remove a disc thoracoscopically decreased as experience was gained by the surgeon. The amount of intraoperative blood loss was comparable using the two methods. There was significantly increased flexibility in the released segments with both techniques, compared with the flexibility in the intact levels for all three loading directions. There was no difference in the motion obtained after release between the two techniques. Conclusion. Open and thoracoscopic anterior release and discectomy have been demonstrated, through biomechanical in vitro testing, to increase the flexibility of the spine to a similar extent.

Subchondral defects in caprine femora augmented with in situ setting hydroxyapatite cement, polymethylmethacrylate, or autogenous bone graft: biomechanical and histomorphological analysis after two-years

Juxta‐articular defects pose significant challenges due to the high risk of fracture of the subchondral plate and articular cartilage. We evaluated the mechanical and histomorphological repair process of caprine subchondral femoral defects augmented with either a bioresorbable in situ setting hydroxyapatite cement (HAC), polymethylmethacrylate (PMMA), autogenous bone graft (AG), or left empty. Twelve‐mm subchondral defects were made bilaterally in the medial femoral condyles of skeletally mature goats and augmented with a test material or left empty. Femurs were harvested at varying time periods out to 2 years and evaluated for subchondral stiffness and histomorphological indices. Several defects augmented using autograft or left empty sustained focal fracture of the subchondral plate. No HAC or PMMA augmented defects showed evidence of subchondral fracture. The HAC and PMMA augmented defects showed comparable stiffness at all time points. The mean volume fraction of HAC remaining within the defects progressively decreased from 96% at 24 h to 38% at 2 years. The new bone replacing the HAC appeared to have normal physiological architecture and orientation. In situ setting hydroxyapatite cement may be a viable alternative for the repair of subchondral defects with an important advantage that while undergoing gradual resorption and replacement with host bone, mechanical integrity of the skeletal defect is maintained.

Thoracoscopic discectomy and fusion in an animal model: Safe and effective when segmental blood vessels are spared

Study Design. Disc–endplate excision and spine fusion were compared in animals randomly assigned to segmental vessel-spared and segmental vessel-ligated groups in an in vivo goat model of anterior spine discectomy and fusion using thoracoscopic techniques. Objectives. To compare safety and completeness of disc and endplate excision, and to perform a histologic and biomechanical comparison between fusion masses when the segmental vessels are spared and when they are ligated using thoracoscopic techniques. Summary off Background Data. Because thoracoscopy is relatively new and technically demanding, many surgeons ligate the segmental blood vessels to enhance spine exposure and limit the risk of injury during discectomy and fusion. Although rare, spinal cord compromise secondary to segmental vessel ligation has been reported. Methods. This study was divided into two phases. In Phase 1, 10 mature goats were randomly assigned to either the segmental vessel - ligated or the segmental vessel-spared group. Disc and endplate excision was performed at six consecutive thoracic levels in each animal (30 levels per group). The animals were killed, and the depth of disc excision was measured in the transverse and sagittal planes. The vertebral bodies then were separated through the disc space; photographic images of the endplates were digitized, and the area of endplate excision was calculated. In Phase 2, 12 mature goats were randomly assigned to the segmental vessel-ligated or vessel-spared group, and five noncontiguous thoracic segments were fused using autologous iliac crest graft. At 4 months the animals were killed, and the spines were harvested. At each disc level, the three-dimensional rotational and translational motions were analyzed and histomorphometric analysis was performed. Results. Phase 1: Each animal survived the operative procedure, and no surgical complications occurred. No difference was found between vessel - ligated and vessel - spared groups in operative time (21.8 vs 22.7 minutes per disc), blood loss (97 vs 159 mL), or transverse (81%vs 74%) or sagittal (85%vs 85%) disc excision. The total area of endplate excision was 70% in the vessel - ligated group and 67% in the vessel - spared group (P > 0.1). Phase 2: Biomechanical testing demonstrated no difference in stiffness of the fused segments between the two groups in flexion–extension or axial rotation. However, greater flexibility in lateral bending was found in the specimens whose vessels were ligated (P < 0.05). The percentage of trabecular bone volume was similar between the two groups. Conclusions. The segmental vessels in the thoracic spine can be effectively spared without injury during disc excision and fusion. Although slightly more disc area was excised with ligation of the vessels, this was not statistically significant, and the fusion mass was similar between the two groups. Sparing the segmental vessels may provide blood supply that aids fusion mass, and the result may be greater spine stiffness in the coronal plane. Sparing the segmental vessels during thoracoscopic anterior disc excision and fusion can be safe. It should be considered in patients with a high risk for neurologic injury because of decreased spinal cord perfusion in revision surgery, severe kyphosis, congenital anomalies. Because the neurologic risk of vessel ligation has not been clearly established for idiopathic scoliosis, the surgeon will have to consider the risk–benefit ratio of adopting these methods when deciding not to ligate vessels in these patients.

Stabilization of a short juxta-articular bone segment with a circular external fixator.

The objective of the current study was to evaluate the stabilization of a simulated juxta-articular bone segment with a circular external fixator, and to determine which method of fixation improved bending stabilization while preserving the axial dynamization of a three-wire configuration. Frames were divided into three groups: wire, half-pin and hybrid and tested in axial compression, torsion, anteroposterior bending and mediolateral bending. Hybrid frames using 4 mm half-pins improved the anteroposterior stabilization of the short bone segment while maintaining axial characteristics similar to a three-wire frame. Increasing the bending stabilization will improve bone segment alignment while permitting axial micromotion beneficial to bone healing.

New rod-plate anterior instrumentation for thoracolumbar/lumbar scoliosis: Biomechanical evaluation compared with dual-rod and single-rod with structural interbody support

Study Design. A new rod-plate anterior implant was designed to provide plate fixation at the cephalad and caudal-end segments of a 5-level anterior spine construct. Biomechanical testing was performed on calf spines instrumented with 5-segment anterior scoliosis constructs. Objectives. To analyze the initial and post-fatigue biomechanical performance of the new implant, and compare it to an anterior dual-rod construct and a single-rod construct with interbody cages. Summary of Background Data. Using single-rod anterior instrumentation for thoracolumbar and lumbar scoliosis, an unacceptable incidence of loss of correction, segmental kyphosis, and pseudarthrosis has been reported. Inadequate construct stiffness due to early postoperative bone-screw interface failure, especially at cephalad and caudal-end vertebrae, has been implicated as the cause of these complications. Methods. Thirty calf spines were instrumented over 5 segments with: (1) single-rod augmented with rod-plate implants, (2) dual-rod construct, and (3) single-rod with titanium mesh cages. Stiffness in flexion-extension and lateral bending modes was determined initially and post-cyclical loading by measuring segmental range of motion (ROM). Post-fatigue screw pullout tests were also performed. Results. In lateral bending, the caudal-end segmental ROM for rod-plate construct was 54% less than single-rod with cages construct (P < 0.05), with no difference between rod-plate and dual-rod constructs. In flexion-extension, the rod-plate construct showed 45% to 91% (initial test) and 84% to 90% (post-fatigue) less ROM than the single-rod with cages construct (P < 0.001). Again, there was no difference between rod-plate and dual-rod constructs at the cephalad and caudal-end segments. Post-fatigue screw pullout strengths of the rod-plate construct were significantly greater than those of the dual-rod and single-rod with cages constructs (P < 0.05). Conclusions. The rod-plate construct was significantly stiffer and provided greater stability of bone-screw interfacethan the single-rod with cages construct. It achieved similar stiffness and improved bone-screw interface stability compared to dual-rod construct.

Mechanical testing of a single rod versus a double rod in a long-segment animal model.

This study involved the mechanical testing of single-rod segmental hook fixation and double-rod segmental hook fixation in a long-segment animal model. The goals were first to compare the flexibility of a single-rod scoliosis construct with that of a double-rod construct when tested in torsion, and second, to determine the effect of not using instrumentation with every vertebral segment for the single rod. Another study found that the single-rod construct was as stiff in torsion as the standard double-rod construct in a model of 10 vertebral segments. The amount of neutral zone (NZ) rotation was tested in five calf spines using an MTS (Material Testing System) machine. Five constructs were tested and included 1) a single rod with hooks at every level except the apex; 2) a single rod with two fewer hooks; 3) a single rod with four fewer hooks; 4) a double-rod construct; and 5) no instrumentation. The amount of NZ rotation between vertebral segments was measured over 12, 10, 8, 6, 4, and 2 vertebral segments. An analysis of variance with all constructs showed that the instrumented spines had significantly less movement than did the uninstrumented spine. Statistical comparison using analysis of variance of constructs (constructs 1 to 4) showed that over 12 vertebral segments (T4-L3), all single-rod constructs (constructs 1 to 3) allowed more NZ rotation than did the standard double-rod construct. This testing indicated that over 12 vertebral segments the single rod allowed more NZ rotation than a double-rod construct.

Mechanical effect of posterior wire or half-pin configuration on stabilization utilizing a model of circular external fixation of the foot.

Background: Numerous studies have addressed biomechanical characteristics of circular external fixation of long bones. The objective of the present study was to evaluate stabilization of a simulated foot model using external fixation with either calcaneal tensioned stopper wires or half-pins. Methods: Fixation configurations of the calcaneus included two parallel wires, two wires crossing at either 30° or 45°, a 4-mm- and 5-mm-diameter single half-pin, or two half-pins inserted at a cross-angle of either 45° or 90°. All frames were tested in axial compression, anteroposterior (AP) bending, and mediolateral (ML) bending. Results: An increase in wire cross-angle improved the axial and AP bending stabilization but had no influence on ML bending. Utilization of a single calcaneal half-pin instead of two cross-wires resulted in a considerable reduction in ML bending stabilization. Frame configurations with two half-pins substantially improved axial and ML bending stabilization. Due to the medial location of the metatarsal wire stopper, an increase in half-pin cross-angle significantly improved ML bending stiffness under lateral foot loading. Under the medial foot loading, however, the half-pin cross-angle had no effect on ML bending stabilization. Replacement of cross-wires with two half-pins significantly improved the AP bending stiffness only when the half-pin cross-angle was reduced to 45°. In all modes of two half-pin frame loading, the half-pin diameter had a substantial effect on foot stabilization. Conclusions: Although the wire cross-angle, half-pin cross-angle, and half-pin diameter affect the stability of foot circular external fixation, the influence of these mechanical parameters on foot stabilization is dependent on the mode and location of loading. Clinical Relevance: The results of the present mechanical testing can be utilized as a useful guideline for the optimization of circular external fixation of the foot.

Mechanical evaluation of a tapered thread-run-out half-pin designed for external skeletal fixation in small animals

An in vitro mechanical study was performed to compare the stiffness, maximum load, and cyclic load-to-failure of a new external fixation half-pin design utilizing a tapered thread-run-out (TRO) feature with currently available positive profile thread (PP) half-pins. Five different sizes of each of the two pin types were evaluated. Under static loading, TRO pins were significantly stiffer and had a higher maximum load compared to the similar sized PP pins (p <0.0001 for all comparisons). In cyclic fatigue testing, TRO pins lasted 2.3- to 4.9-fold more cycles than PP pins of similar size (p <0.0001 for all comparisons). The increased pin stiffness and improved cyclic lifespan provided by TRO pins may be especially valuable in the stabilization of biologically and mechanically challenging fracture cases where healing is prolonged.