Antonio Valdevit

A biomechanical comparison of posterior cruciate ligament reconstruction techniques.

Most posterior cruciate ligament reconstruction techniques use both tibial and femoral bone tunnels for graft placement. Because of the acute angle the graft must make to gain entrance into the tibial tunnel, abnormal stresses are placed on the graft that could lead to graft failure. An alternative technique for posterior cruciate ligament reconstruction involves placement of the bone plug from the graft anatomically on the back of the tibia (inlay), preventing formation of an acute angle at the tibial attachment site. We used six pairs of human cadaver knees to compare the biomechanical properties of these two techniques. One knee from each pair underwent tunnel reconstruction while the other knee underwent inlay reconstruction. There was significantly less anterior-posterior laxity in the inlay group when compared with the tunnel group from 30° to 90° of knee flexion and after repetitive loading at 90° of knee flexion. Evaluation of the grafts revealed evidence of mechanical degradation in the tunnel group but not in the inlay group. The inlay technique resulted in less posterior translation with less graft degradation than did the tunnel technique for posterior cruciate ligament reconstruction.

Spine Fusion Using Cell Matrix Composites Enriched in Bone Marrow-Derived Cells

Bone marrow-derived cells including osteoblastic progenitors can be concentrated rapidly from bone marrow aspirates using the surface of selected implantable matrices for selective cell attachment. Concentration of cells in this way to produce an enriched cellular composite graft improves graft efficacy. The current study was designed to test the hypothesis that the biologic milieu of a bone marrow clot will significantly improve the efficacy of such a graft. An established posterior spinal fusion model and cancellous bone matrix was used to compare an enriched cellular composite bone graft alone, bone matrix plus bone marrow clot, and an enriched bone matrix composite graft plus bone marrow clot. Union score, quantitative computed tomography, and mechanical testing were used to define outcome. The union score for the enriched bone matrix plus bone marrow clot composite was superior to the enriched bone matrix alone and the bone matrix plus bone marrow clot. The enriched bone matrix plus bone marrow clot composite also was superior to the enriched bone matrix alone in fusion volume and in fusion area. These data confirm that the addition of a bone marrow clot to an enriched cell-matrix composite graft results in significant improvement in graft performance. Enriched composite grafts prepared using this strategy provide a rapid, simple, safe, and inexpensive method for intraoperative concentration and delivery of bone marrow-derived cells and connective tissue progenitors that may improve the outcome of bone grafting.

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out.

BACKGROUND CONTEXTnBiomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better fit and fill of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion.nnnPURPOSEnTo determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion.nnnSTUDY DESIGN/SETTINGnThis biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format.nnnMETHODSnPorcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Students t test.nnnRESULTSnPullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion.nnnCONCLUSIONSnConical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.

Articular Cartilage Contact Pressure after Tibial Tuberosity Transfer: A Cadaveric Study

Medial transfer of the tibial tuberosity has been commonly used for treatment of recurrent dislocation of the patella and patellofemoral malalignment. In this study, six fresh human cadaveric knees were used. Static intrajoint loads were recorded using Fuji Prescale pressure-sensitive film for contact pressure and contact area determination in a closed kinetic chain knee testing protocol. Peak pressures, average contact pressures, and contact areas of the patellofemoral and tibiofemoral joints were calculated on native intact knee specimens and after tibial tuberosity transfer. All native intact knee specimens had a normal Q angle. Medialization of the tibial tuberosity significantly increased the patellofemoral contact pressure. Medial displacement of the tibial tuberosity also significantly increased the average contact pressure of the medial tibiofemoral compartment and changed the balance of tibiofemoral joint loading. The results of our study suggest that caution should be used when transferring a patellar tendon in the face of a preexisting normal Q angle as this will result in abnormally high peak pressure within the tibiofemoral joint. Overmedialization of the tibial tuberosity should be avoided in the varus knee, the knee after medial meniscectomy, and the knee with preexisting degenerative arthritis of the medial compartment.

Selective retention of bone marrow-derived cells to enhance spinal fusion.

Connective tissue progenitors can be concentrated rapidly from fresh bone marrow aspirates using some porous matrices as a surface for cell attachment and selective retention, and for creating a cellular graft that is enriched with respect to the number of progenitor cells. We evaluated the potential value of this method using demineralized cortical bone powder as the matrix. Matrix alone, matrix plus marrow, and matrix enriched with marrow cells were compared in an established canine spinal fusion model. Fusions were compared based on union score, fusion mass, fusion volume, and by mechanical testing. Enriched matrix grafts delivered a mean of 2.3 times more cells and approximately 5.6 times more progenitors than matrix mixed with bone marrow. The union score with enriched matrix was superior to matrix alone and matrix plus marrow. Fusion volume and fusion area also were greater with the enriched matrix. These data suggest that the strategy of selective retention provides a rapid, simple, and effective method for concentration and delivery of marrow-derived cells and connective tissue progenitors that may improve the outcome of bone grafting procedures in various clinical settings.

Biomechanical comparison of antegrade and retrograde nailing of humeral shaft fracture.

A pair controlled study was conducted to compare biomechanical properties of antegrade and retrograde nailing of humeral fractures. First, six paired fresh anatomic specimen humeri were used to compare the properties of humeri fractured at the middle to distal diaphyses junction that were nailed from the retrograde approach with the Humeral Locked nail with those of contralateral intact humeri. An 18 additional pairs were divided into three equal groups by distal, proximal, or middiaphysis location of a standardized 5-mm bone defect to simulate unstable fractures. The retrograde and antegrade nailings were performed in each pair in a random manner. Nail and bone constructs were tested for bending stiffness by nondestructive three-point bending and for torsional stiffness by destructive torsional tests. Compared with intact humeri, fractured humeri fixed with nails had 28.6% posteroanterior and 31.4% mediolateral bending stiffness, 22.5% torsional stiffness, and 43.3% failure torque. For distal fractures, retrograde nailing showed significantly more initial stability and higher bending and torsional stiffness; for proximal fractures, antegrade nailing showed similar properties. For middle to distal diaphyses junction fractures, retrograde and antegrade nailing were indistinguishable. The defect created as an entry portal for retrograde nailing reduced the bone strength only 11.1%. These results suggest that retrograde nailing, which is less detrimental to shoulder function than is antegrade nailing, is an acceptable alternative treatment for humeral shaft fractures. In addition, nailing from the short to the long bone segments can improve mechanical properties of the fixation construct because of better nail and bone interface purchase.

The Effects of Tibial Rotation on Posterior Translation in Knees in Which the Posterior Cruciate Ligament Has Been Cut

Background: One of the most useful clinical tests for diagnosing an isolated injury of the posterior cruciate ligament is the posterior drawer maneuver performed with the knee in 90° of flexion. Previously, it was thought that internally rotating the tibia during posterior drawer testing would decrease posterior laxity in a knee with an isolated posterior cruciate ligament injury. In this study, we evaluated the effects of internal and external tibial rotation on posterior laxity with the knee held in varying degrees of flexion after the posterior cruciate and meniscofemoral ligaments had been cut. Materials and Methods: Twenty cadaveric knees were used. Each knee was mounted in a fixture with six degrees of freedom, and anterior and posterior forces of 150 N were applied. The testing was conducted with the knee in 90°, 60°, 30°, and 0° of flexion with the tibia in neutral, internal, and external rotation. All knees were tested with the posterior cruciate and meniscofemoral ligaments intact and transected. Repeated-measures analysis of variance was used for statistical analysis. Results: At 30°, 60°, and 90° of flexion, there was a significant increase in posterior laxity following transection of the posterior cruciate and meniscofemoral ligaments. At 60° and 90° of flexion, there was significantly less posterior laxity when the tibia was held in internal compared with external rotation. At 0° and 30° of flexion, there was no significant difference in posterior laxity when the tibia was held in internal compared with external rotation. Conclusions: After the posterior cruciate and meniscofemoral ligaments had been cut, posterior laxity was significantly decreased by both internal and external rotation of the tibia. Internal tibial rotation resulted in significantly less laxity than external tibial rotation did at 60° and 90° of knee flexion. Clinical Relevance: An isolated injury of the posterior cruciate ligament is best detected when a posterior drawer test is performed with the knee in 90° of flexion. Repeating this test with the tibia internally rotated will result in a substantial decrease in the amount of posterior laxity at 60° and 90° of knee flexion.

Distribution of patellofemoral joint pressures after femoral trochlear osteotomy.

Anterior osteotomy of the lateral femoral condyle was designed for the treatment of recurrent patellar dislocations. This study examined whether anterior femoral trochlear osteotomy significantly elevates contact pressures in the patellofemoral joint. Static intrajoint loads and contact area determination were recorded using prescale Fuji pressure-sensitive film under static loading. Peak pressures, average pressures and contact areas of the patellofemoral joint were calculated on intact specimens and after anterior osteotomy of the lateral condyle. Our results indicate that a 6-mm or 10-mm anterior osteotomy of the lateral condyle significantly elevates patellofemoral contact pressures.

A Comparison of Preoperative Imaging Techniques for Predicting Patellar Tendon Graft Length before Cruciate Ligament Reconstruction

Autogenous bone-patellar tendon-bone is commonly used as graft material for cruciate ligament reconstructions. If this type of graft is too long, graft fixation other than an interference screw may be required. If it is too short, selection of another type of graft may be necessary. If the length of the patellar tendon portion of the graft could be accurately predicted, preoperative planning could determine the adequacy of this graft and choice of fixation for the planned procedure. Using lateral radiographs with the knee flexed 30°, standard magnetic resonance imaging, and magnetic resonance imaging with supplemental three-dimensional reconstructions, we measured the length of the patellar tendon in cadavers and then compared these measurements with the actual patellar tendon length measurements. Linear regression analysis resulted in r2 values of 0.80, 0.43, and 0.65 for lateral radiographs, standard magnetic resonance imaging, and magnetic resonance imaging with three-dimensional reconstructions, respectively. We concluded that lateral radiographs at 30° of knee flexion are the most accurate predictors of patellar tendon length.

Cranial defect repair using e-PTFE: part I. Evaluation of bone stiffness.

Autologous bone grafts are the preferred material for craniofacial reconstruction, but such procedures lead to increased operative time and bleeding, donor site morbidity, and graft resorption. The efficacy of expanded-polytetrafluoroethylene (e-PTFE) sheets to increase bone regeneration and remodeling in cranial defects using a rabbit model was evaluated by mechanical testing. New Zealand white rabbits were divided into 3 groups and sacrificed 6 months after surgery. In the Split Table group, (n = 16), a bilateral bone defect was created on the outer table of the parietal bones. In the Full Table group, (n = 16), a bilateral defect was created through both the inner and outer table of the cranium. The control group, (n = 10) was subjected to a sham operation. Indentation testing was performed to determine the stiffness of newly formed bone in and around the defect. Near the center of the defect, Split Table defects repaired with e-PTFE resulted in significantly stiffer bone than regenerated control bone. The Full Table defects repaired with e-PTFE also resulted in bone significantly stiffer than control regenerated bone around the central region of the defect. The data supports the hypothesis that e-PTFE improves the repair of cranial defects in a rabbit model. It is surmised that the porosity of the e-PTFE provides a stable scaffold for migration of tissue regenerating cells, which may be preferentially localized near the cranial suture lines. This porosity may also provide a barrier to fibrous tissue regenerating cells.