The effects of modified posterior tibial slope on anterior cruciate ligament strain and knee kinematics: a human cadaveric study.

Increases to the posterior tibial slope can lead to an anterior shift in tibial resting position. However, the effect of this shift on anterior cruciate ligament (ACL) strain has not been investigated sufficiently. This study examined the relationship between increased tibial slope and ACL strain, as well as the subsequent kinematics of the tibiofemoral joint. We hypothesized increases in slope would shift the tibia anterior relative to the femur and increase ACL strain. Anterior cruciate ligament strain measurements and tibiofemoral kinematics were compared for 5 intact and experimental knees subject to anterior opening wedge osteotomy. Combinations of both compressive and AP loading were applied. As slope increased, the resting position of the tibia shifted anteriorly, external tibial rotation increased, and tibial translation remained unchanged. Contrary to our hypothesis, ACL strain decreased. The clinical implication of these findings is that alterations to the posterior tibial slope should not increase strain in the ACL.

Torsional stability of cross-link configurations: a biomechanical analysis.

BACKGROUND CONTEXT: Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known. PURPOSE: The purpose of this study was to compare the torsional stability of three different cross-link configurations. STUDY DESIGN: Biomechanical analysis of segmental instrumentation constructs using porcine spines. METHODS: Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences. RESULTS: Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01). CONCLUSIONS: The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.

A biomechanical comparison of posterior cruciate ligament reconstructions using single- and double-bundle tibial inlay techniques.

BACKGROUND: The efficacy of using a double-bundle versus single-bundle graft for posterior cruciate ligament reconstruction has not been demonstrated. HYPOTHESIS: A double-bundle graft restores knee kinematics better than a single-bundle graft does in tibial inlay PCL reconstructions. STUDY DESIGN: Controlled laboratory study. METHODS: Eight cadaveric knees were subjected to 6 cycles from a 40-N anterior reference point to a 100-N posterior translational force at 10 degrees , 30 degrees , 60 degrees , and 90 degrees of flexion. Testing was performed for the intact and posterior cruciate deficient knee as well as for both reconstructed conditions. Achilles tendons, divided into 2 equal sections, were prepared as both single-bundle and double-bundle grafts. Both grafts were employed in the same knee, and the order of graft reconstruction was randomized. RESULTS: There were no statistical differences in translation between the intact state and either of the reconstructions (P > .05) or between either of the reconstructions at any flexion angle (P > .05). CONCLUSION: No differences in translation between the 2 graft options were identified. CLINICAL RELEVANCE: The use of a double-bundle graft may not offer any advantages over a single-bundle graft for tibial inlay posterior cruciate reconstructions.

The effect of humeral component anteversion on shoulder stability with glenoid component retroversion.

BACKGROUND: Posterior glenoid bone loss is often seen in association with glenohumeral osteoarthritis. This posterior asymmetric wear can lead to retroversion of the glenoid component and posterior instability after total shoulder arthroplasty. Options for the treatment of this asymmetric wear include eccentric reaming of the so-called high side, bone-grafting, and/or anteverting the humeral component. Although anteverting the humeral component has been advocated by many, it has not been substantiated on the basis of biomechanical data. The purpose of the present study was to determine whether anteverting the humeral component increases the stability of a total shoulder replacement with a retroverted glenoid component. METHODS: A total shoulder arthroplasty was performed in eight human cadaveric shoulders. The glenoid component was placed in 15 degrees of retroversion. Two humeral versions were tested for each specimen: anatomic version and 15 degrees of anteversion relative to anatomic version. The specimens were mounted supine in a custom fixture on a servohydraulic testing system. The humerus was translated posteriorly by one-half of the width of the glenoid. Three positions of humeral rotation were tested for each position of humeral version. Both the energy and the peak load were analyzed as measures of joint stability. RESULTS: There was no significant difference in either energy or peak load between the tests performed with the humeral component in 15 degrees of anteversion and those performed with the component in anatomic version in any of the three rotational positions (p > 0.05). CONCLUSIONS: Although anteverting the humeral component during total shoulder arthroplasty to compensate for glenoid retroversion has been advocated, these data suggest that compensatory anteversion of the humeral component does not increase the stability of a shoulder replacement with a retroverted glenoid component.

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.

Spatial organization of types I and II collagen in the canine meniscus.

The meniscus of the knee joint is a fibrocartilage mainly composed of type I collagen and smaller amounts of type II collagen. The distribution of type II collagen in the canine meniscus and its spatial relationship to type I collagen was examined by immunohistochemistry and confocal microscopy. Dorsal and coronal slices of the mid-section of medial and lateral menisci from the knee joints of skeletally mature dogs were predigested with Streptomyces hyaluronate lyase and bacterial Protease enzyme XXIV. Monoclonal antibodies against type I collagen (CP17L) and type II collagen (II-II6B3) and an anti-type II collagen polyclonal antibody (AB759) were employed. The staining for type II collagen in the extracellular matrix of hyaline articular cartilage was diffuse without any identifiable spatial organization. In striking contrast, type II collagen in the fibrocartilage of the meniscus stained as an organized network. Type II collagen was distributed throughout the meniscus with the exception of the outer zone containing the blood vessels. Coronal and dorsal staining of the meniscus showed bundles of circumferential fibrils of type I that colocalized with type II collagen in specific sites. These bundles were enwrapped in a second organizational fibrillar system of types I and II collagen that also colocalized. Bundles of circumferential fibrils appeared in cross-section in coronal sections as dots within the interstitial spaces framed by the network of types I and II collagen of the second system. Confocal overlays showed that types I and II collagens were superimposed, suggesting a close spatial proximity between the two collagens. The cells were confined to the types I and II collagen fibrils that enwrapped the bundles. A striking feature of the radial tie fibers was patches of type II collagen without colocalized type I collagen. Our study reveals a unique network of type II collagen in fibrocartilage of the meniscus that serves as a morphological distinction between fibro- and hyaline cartilage.

Comparative evaluation of the osteoinductivity of two formulations of human demineralized bone matrix.

In the United States, demineralized bone matrix (DBM) is considered a transplantable tissue and therefore is regulated primarily by the American Association of Tissue Banks. Even though DBM is not subjected to the same regulations relative to performance claims as medical devices are, one would expect different processing methods might yield DBM preparations of different osteoinductive potential. The purpose of this study was to use an established athymic rat model to compare the osteoinductive properties of two commercially available human DBMs prepared using different methods but having essentially identical product claims. Sixteen female athymic rats were used to test equivalent volumes of two lots each of Grafton Putty (Osteotech, Inc., Eatontown, NJ), Osteofil (Regeneration Technologies, Inc., Alachua, FL), and rat DBM. At 28 days after implantation, qualitative and semiquantitative microscopy showed no significant differences in bone formation between the two lots from each source, but rat DBM produced significantly more bone than Grafton, which produced significantly more bone than Osteofil. Our results suggest that methods of graft processing may represent a greater source of variability than do differences among individual donors. Whether these differences relate to methods of demineralization, carrier, dose of DBM per volume, or to some other factor remains to be determined.

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.

Non-linear fitting of mechanical data for efficacy determination of single versus double bundle Achilles tendon grafts for PCL reconstructions.

The Posterior Cruciate Ligament (PCL) consists of two primary bundles and is the primary restraint to posterior tibial translation. The emergence of the Posterior Tibial Inlay Technique (PTIT) has demonstrated a mechanical advantage over the commonly used Tibial Tunnel Method (TTM). This study was designed to investigate the effects on knee mechanics from both a Single-bundle (SB) and Double-bundle (DB) Achilles tendon PCL reconstruction using a PTIT. The investigators hypothesized that a DB graft demonstrates a mechanical advantage compared to the SB graft. A materials testing machine provided six sequential loading cycles of 100 N while total translation was measured. The testing was conducted at 90 degrees, 60 degrees, 30 degrees and 10 degrees of flexion in neutral tibial rotation. While the mechanical advantages of the DB graft were detected, specifically at small laxity values and in the physiological range of normal gait, the additional complexity of the technique may clinically outweigh the mechanical advantages.

In vitro stability of FRA spacers with integrated crossed screws for anterior lumbar interbody fusion.

STUDY DESIGN: Mechanical testing of human lumbar functional spine units was carried out after instrumenting the disc space with femoral ring allografts (FRAs) with and without integrated crossed anterior screws applied into the adjacent bodies. OBJECTIVES: To assess the stability of FRA construct with and without the integrated crossed anterior screws and to compare that with the intact specimen. SUMMARY OF BACKGROUND DATA: Most modern methods of achieving anterior lumbar intervertebral fusion rely on the use of interbody spacers to restore and maintain intervertebral height, overall alignment, and stability while facilitating arthrodesis. The FRAs have the advantage of biologic compatibility but may not have enough stability when used as stand-alone devices. FRA spacers alone are less stiff in torsion and extension compared with other instrumented constructs. Increased motion could lead to higher failure rates because of graft migration and pseudarthrosis. This makes the use of supplementary anterior or posterior fixation necessary. The current authors hypothesized that the addition of anterior integrated crossed screws applied through the FRA spacer into the adjacent vertebral bodies would increase the stability of the FRA spacer in extension and torsion in the absence of further posterior instrumentation. METHODS: Seven fresh-frozen human cadaveric lumbar spine functional spine units were tested applying cantilever bending moments in flexion, lateral bending, torsion, and extension. The specimens were tested in the following sequence: intact, with FRA spacer alone and with FRA spacer and integrated crossed screws. The stiffness of each construct was then compared with the intact specimen. Specimens with obvious deformity on radiographs or dual-energy radiograph absorptiometry t score values <-1.2 were excluded. RESULTS: The addition of the integrated crossed metal screw system improved the stiffness of the construct by 53% over the intact (P = 0.02) and by 31% over the FRA alone in extension (P = 0.01), whereas it improved the stiffness by 40% over the intact (P = 0.03) and by 18% over the FRA alone in torsion (P = 0.21). The crossed screw system did not improve the stiffness compared with intact in either flexion or lateral bending modes. CONCLUSIONS: Although there is a trend toward improved stability of the FRA spacers in torsion with the addition of the metal screws, this is not statistically significant. The integrated crossed anterior metal screw system significantly improves the stability of the FRA spacers in extension when used for anterior lumbar interbody fusion.

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.