Kevin A. Thomas

Pressure-volume characteristics of the intact and disrupted pelvic retroperitoneum.

Hemorrhage is a major cause of mortality in pelvic fractures. Bleeding can be controlled in hypotensive patients by direct ligation, angiographic embolization, pelvic packing, and acute external fixation. Acute application of an external fixator can reduce pelvic volume and reduce bleeding fractures to effect tamponade. This therapy assumes that the pelvis represents a closed space, which clearly is not true anatomically. However, the premise may hold functionally. This study explored the relationship between pressure and volume in the intact and disrupted pelvic retroperitoneum. In cadaveric specimens, the external iliac vein was dissected, ruptured, and cannulated. This method allowed controlled flow of fluid, with simultaneous measurement of pressure, into the intact retroperitoneum. Open book pelvic fractures were created by applying external rotation to the pelvis through the femoral heads. The pressure-volume measurements, without and with external fixation applied, were repeated after the fracture, as well as after a laparotomy. In the intact retroperitoneum, pressures rapidly rose to an average of 30 mm Hg after infusion of 5 liters of fluid. After fracture, up to 20 liters of fluid could be infused at pressures not exceeding 35 mm Hg. External fixation increased pressures approximately 3 mm Hg at low fluid volumes, and approximately 11 mm Hg at the highest fluid volumes. Laparotomy decreased retroperitoneal pressure from approximately 35 mm Hg to approximately 15 mm Hg. The results of the study suggest that low-pressure venous hemorrhage may be tamponaded by an external fixator, given that enough fluid volume is present in the pelvic retroperitoneum. However, external fixation may not generate sufficient pressure to stop arterial bleeding. In any case, it seems that a large volume of fluid must be lost into the pelvis before an external fixator can have much effect on retroperitoneal pressures.

Evaluation of methods of internal fixation of transverse patella fractures: a biomechanical study.

Biomechanical testing was performed to evaluate five techniques of internal fixation of transverse patella fractures. Using cadaveric lower extremities, transverse osteotomies of the patella were performed, and the simulated fractures were fixed with the following techniques: the modified tension band, anterior tension band with a supplemental cerclage wire (the Pyrford technique), tension band with cancellous bone screws, Pyrford technique with cancellous screws, and cancellous screws alone. The fixation techniques were evaluated by measuring the separation of the fracture fragments during loading to produce a physiologic range of motion (90° flexion to full extension). All techniques functioned adequately, with no fracture gap exceeding 1 mm. The tension band with screws technique performed significantly better than did the modified tension band, with an average fracture gap approximately half that of the traditional modified tension band technique. Mechanically, the addition of the screws to the tension band techniques reduces fracture separation by providing compression throughout the range of motion and by resisting the tensile loading during terminal extension.

The Outcomes of Two Knee Scoring Questionnaires in a Normal Population

The Noyes and Lysholm knee scoring questionnaires, commonly used for follow-up assessment after knee surgery, were developed based on knees with preexisting pathologic changes and have not been standardized to normal knees. We administered both questionnaires to normal subjects. Any subject reporting a history of injury or surgery to either knee, or preexisting knee pathologic changes, was excluded. From a total of 492 knees evaluated, 418 knees (253 male, 165 female) qualified for statistical analysis. The average age of the group was 17.6 years (range, 13 to 25). For male subjects, the total Noyes and Lysholm scores averaged 99.10 (range, 68 to 100) and 99.10 (range, 77 to 100), respectively. For female subjects, the average Noyes and Lysholm scores were 97.82 (range, 72 to 100) and 97.16 (range, 75 to 100), respectively. The 95% confidence interval computed for each of these groups did not contain the maximal value of 100. The female athletes reported significantly lower total scores than the male athletes on both questionnaires. For the Lysholm questionnaire, the male athletes scored significantly lower than the maximum in all categories except support and stair climbing, and the female athletes scored significantly lower than the maximum in all categories except limp and thigh atrophy. The range of scores found in this highly selected, “normal” population exemplifies the need for more accurate instruments in the evaluation of knee surgical outcomes.

Bioresorbable polylactide interbody implants in an ovine anterior cervical discectomy and fusion model: three-year results.

Study Design. In vivo study of anterior discectomy and fusion using a bioresorbable 70:30 poly(l-lactide-co-d,l-lactide) interbody implant in an ovine model. Objective. To evaluate the efficacy of the polylactide implant to function as an interbody fusion device, and to assess the tissue reaction to the material during the resorption process. Summary of Background Data. The use of polylactide as a cervical interbody implant has several potential advantages when compared with traditional materials. Having an elastic modulus very similar to bone minimizes the potential for stress shielding, and as the material resorbs additional loading is transferred to the developing fusion mass. Although preclinical and clinical studies have demonstrated the suitability of polylactide implants for lumbar interbody fusion, detailed information on cervical anterior cervical discectomy and fusion (ACDF) with polylactide devices is desirable. Methods. Single level ACDF was performed in 8 skeletally mature ewes. Bioresorbable 70:30 poly (l-lactide-co-d,l-lactide) interbody implants packed with autograft were used with single-level metallic plates. Radiographs were made every 3 months up to 1 year, and yearly thereafter. The animals were killed at 6 months (3 animals), 12 months (3 animals), and 36 months (2 animals). In addition to the serial plain radiographs, the specimens were evaluated by nondestructive biomechanical testing and undecalcified histologic analysis. Results. The bioresorbable polylactide implants were effective in achieving interbody fusion. The 6-month animals appeared fused radiographically and biomechanically, whereas histologic sections demonstrated partial fusion (in 3 of 3 animals). Radiographic fusion was confirmed histologically and biomechanically at 12 months (3 of 3 animals) and 36 months (2 of 2 animals). A mild chronic inflammatory response to the resorbing polylactide implant was observed at both 6 months and 12 months. At 36 months, the operative levels were solidly fused and the implants were completely resorbed. No adverse tissue response was observed in any animal at any time period. Conclusion. Interbody fusion was achieved using bioresorbable polylactide implants, with no evidence of implant collapse, extrusion, or adverse tissue response to the material. The use of polylactide as a cervical interbody device appears both safe and effective based on these ACDF animal model results.

The effects of simulated transverse, anterior column, and posterior column fractures of the acetabulum on the stability of the hip joint.

BACKGROUND Knowledge of the location of the weight-bearing portion of the acetabulum would assist orthopaedic surgeons in the treatment of acetabular fractures. With use of controlled experimental transverse, anterior column, and posterior column osteotomies, we investigated the weight-bearing region of the acetabulum. METHODS Twenty-four fresh-frozen cadaveric hip joints were tested. Simulated transverse fractures were evaluated in twelve specimens, and simulated anterior column and posterior column fractures were tested in six specimens each. Each femur and acetabulum was potted and mounted in an aluminum fixture, with the acetabulum positioned in 25 degrees of flexion and 20 degrees of abduction. Each specimen was tested intact and after successive osteotomies. The transverse osteotomies had roof-arc angles of 60, 50, 40, and 30 degrees. The anterior column and posterior column osteotomies were classified as very low, low, intermediate, or high. Compressive loading to 800, 1200, and 1600 newtons was performed four times for each intact specimen and after each osteotomy. A specimen was considered to be stable if no gross dislocation occurred during any of the four loading cycles. Translation of the femur within the acetabulum also was measured during each trial. RESULTS The number of stable specimens decreased both with higher applied loads and with more superior osteotomies. The stability of the hip was significantly affected by both the location of the fracture and the magnitude of the applied load (p < 0.00005). Translation of the femur within the acetabulum increased with higher applied loads and with more superior osteotomies. CONCLUSIONS Fractures that have a medial roof-arc angle of 45 degrees or less, an anterior roof-arc angle of 25 degrees or less, or a posterior roof-arc angle of 70 degrees or less cross the weight-bearing portion of the acetabulum and necessitate operative treatment.

The effect of divergent screw placement on the initial strength of plate-to-bone fixation.

BACKGROUND Numerous implants exist that allow screws to be placed at varying angles for lag fixation or to fix additional fragments. This study determined how placing screws at different angles affects fixation strength. METHODS Using a bone model, we first investigated the pullout strength of screws inserted at varying angles; then, we studied the strength of plate-bone constructs with end screws placed at divergent angles. RESULTS Varying the screw angle from 0 to 10 to 20 degrees progressively weakened the screw pullout resistance. No additional decrease was found in varying the angle further. In contrast, the strength of fixation of plate to bone was higher for constructs with screws placed at 20 or 30 degrees off of perpendicular when tested in gap-open bending and axial compression (all p < 0.05). No such differences were found in torsion. CONCLUSION The pullout strength with angled screws is reduced, but this does not translate into reduced strength of the bone-to-plate interface.

In vitro comparison of elongation of the anterior cruciate ligament and single- and dual-tunnel anterior cruciate ligament reconstructions

This study evaluated strain in the normal anterior cruciate ligament (ACL) and compared it to four different double-strand hamstring tendon reconstructive techniques. Seventeen fresh-frozen knees from 11 cadavers were tested. The strain in the anteromedial and posterolateral bands of the native ACL and their equivalents in four autograft techniques were measured using differential variable reluctance transducers. The anteromedial band of the intact ACL shortened from 0 degree -30 degrees of flexion, then lengthened to 120 degrees; the posterolateral band of the intact ACL shortened from 0 degree - 120 degrees of flexion. Following ACL excision, these knees underwent reconstruction with double-strand hamstring tendons with either single tibial and femoral tunnels, single tibial and dual femoral tunnels, dual tibial and single femoral tunnels, or dual tibial and dual femoral tunnels. With the exception of the dual-band, dual-tunnel technique, all of the procedures placed greater strain on the reconstructive tissues than was observed on the native ACL, after approximately 30 degrees of flexion. These results indicate that dual-band hamstring tendon reconstructions placed with single tibial and femoral tunnels do not address the complexity of the entire ACL. Rather, these procedures appear to only duplicate the effect of the anteromedial band, while perhaps overconstraining the joint as a result of its inability to reproduce the function of the posterolateral band. During rehabilitation following ACL reconstruction, therefore, only from 0 degree - 30 degrees of the graft tissues are not significantly strained. Dual tibial and femoral tunnel techniques should be evaluated further to more closely recreate knee kinematics following ACL reconstruction.

Biomechanical analysis of nonreamed tibial intramedullary nailing after simulated transverse fracture and fibulectomy

A tibial shaft fracture model was created to study the effects of an intramedullary nail and partial fibulectomy on fracture loading. Cadaveric lower extremities were instrumented with strain gages and subjected to biomechanical testing. A nonreamed nail was inserted into each tibia using only the proximal locking screws. Each specimen was tested under six conditions: intact tibia; intact tibia with nail; fractured tibia with nail removed and fibula intact; fractured tibia with nail and fibula intact; fractured tibia with nail and partial fibulectomy; and fractured tibia with partial fibulectomy and nail removed. In the intact tibia the anterior cortices were in relative tension compared with the posterior cortex. After transverse fracture this relative tension was increased. Inserting the nail after fracture significantly increased anteromedial and anterolateral compressive strains and decreased posterior strains. Performing a partial fibulectomy in the fractured tibia with a nail had no significant effect on the strain patterns. These results confirm the relative anterior tension present in the intact tibia and demonstrate an increase in this anterior tension following transverse fracture. Performing a partial fibulectomy or inserting an intramedullary nail increased anterior compressive loading. This loading alteration may be responsible for the clinical success seen using these treatment methods.

The effect of anterior thoracolumbar plate application on the compressive loading of the strut graft.

Study Design In vitro biomechanical testing was performed using a simulated vertebral body bone graft instrumented with strain gauges. Strains were recorded from various locations on the graft during axial compressive loading. Comparisons were made of the strain patterns recorded before and after application of two different anterior plates. Objectives To quantify the changes in axial compression experienced by the strut graft in the presence of an anterior plate. Summary of Background Data The use of anterior instrumentation to augment anterior thoracolumbar grafting offers the potential advantage of saving additional motion segments while being performed in a single‐stage surgery. Several biomechanical studies have compared the anteriorly grafted and instrumented spine to the compressive axial stiffness and torsional rigidity of the intact spine. No previous study has addressed the loading patterns experienced by the graft before and after plating. Methods Anterior spinal plates of two designs (Amset ALPS Anterior Locking Plate System; AMS, Hayward, California; Anterior Thoracolumbar Locking Plate System; Synthes Spine, Paoli, Pennsylvania) were evaluated to determine the axial compressive forces experienced by the bone graft before and after application of the plates. Bovine spines harvested from 8‐ to 12‐week‐old calves were used for testing. All plates were tested in axial compression to 500 N. Simultaneous recordings were made of the axial strains on the simulated bone graft and the load applied to the construct. Comparisons were made of the strain through the graft without any instrumentation (graft alone, or baseline), with the plate applied, and after removal of the caudal screws (dynamization). Results With the application of the ALPS plate, the compressive strain through the graft adjacent to the plate averaged 77% of the graft alone construct (range, 39% to 158%). After application of the Synthes plate, the compressive strain through the graft adjacent to the plate averaged 34% of the graft alone construct (range, ‐14% to 97%). Once the caudal screws were removed, the dynamized construct allowed near‐baseline compression through the graft. Conclusions Although the literature indicates that the anteriorly instrumented spine may restore the overall spinal structure to near‐normal resistance to rotation and flexion forces, the current study demonstrates that a significant amount of compression through the graft is lost by its application. This decreased axial loading further supports the concept of device‐related osteopenia observed clinically with such devices.

Bioresorbable film for the prevention of adhesion to the anterior spine after anterolateral discectomy

BACKGROUND CONTEXT The development of scar tissue and adhesions postoperatively is a natural consequence of healing but can be associated with medical complications and render reoperation difficult. Many biocompatible products have been evaluated as barriers or deterrents to adhesions. PURPOSE To evaluate the efficacy of a bioresorbable polylactide film as a barrier to adhesion formation after anterolateral discectomy. STUDY DESIGN Experimental study. METHODS Seven, skeletally mature female sheep underwent a retroperitoneal approach to the anterolateral lumbar spine. A discectomy was performed at two levels with an intervening unoperated disc site. One site was treated with a polylactide film barrier (Hydrosorb Shield; MacroPore Biosurgery, San Diego, CA) affixed with tacks manufactured from the same material. The second site was left untreated. Treatment and control sites were randomly assigned. Postmortem analysis included scar tenacity scoring on five spines and histological evaluation on two spines. RESULTS The application of the Hydrosorb film barrier allowed a definite dissection plane during scar tenacity scoring and there was a significant difference in the development of adhesions to the disc between the control and treated sites. Histological evaluation revealed evidence of barrier formation to scar tissue and no significant adverse inflammatory reactions. CONCLUSIONS Hydrosorb Shield appears to be an effective postoperative barrier to scar tissue adhesion after anterolateral discectomy. The use of polylactide tacks was beneficial to affix the barrier film in place. Safety issues associated with delayed healing or adverse response to the film or tacks were not observed. Hydrosorb film may be useful as an antiadhesion barrier facilitating dissection during surgical revision in anterior approaches to the spine. Further studies are indicated to evaluate the performance of the bioresorbable material as an antiadhesion barrier in techniques of spinal fusion and disc replacement.