Novel technique for repairing posterior medial meniscus root tears using porcine knees and biomechanical study.

Transtibial pullout suture (TPS) repair of posterior medial meniscus root (PMMR) tears was shown to achieve good clinical outcomes. The purpose of this study was to compare biomechanically, a novel technique designed to repair PMMR tears using tendon graft (TG) and conventional TPS repair. Twelve porcine tibiae (n = 6 each) TG group: flexor digitorum profundus tendon was passed through an incision in the root area, created 5 mm postero-medially along the edge of the attachment area. TPS group: a modified Mason-Allen suture was created using no. 2 FiberWire. The tendon grafts and sutures were threaded through the bone tunnel and then fixed to the anterolateral cortex of the tibia. The two groups underwent cyclic loading followed by a load-to-failure test. Displacements of the constructs after 100, 500, and 1000 loading cycles, and the maximum load, stiffness, and elongation at failure were recorded. The TG technique had significantly lower elongation and higher stiffness compared with the TPS. The maximum load of the TG group was significantly lower than that of the TPS group. Failure modes for all specimens were caused by the suture or graft cutting through the meniscus. Lesser elongation and higher stiffness of the constructs in TG technique over those in the standard TPS technique might be beneficial for postoperative biological healing between the meniscus and tibial plateau. However, a slower rehabilitation program might be necessary due to its relatively lower maximum failure load.

Concave polyethylene component improves biomechanical performance in lumbar total disc replacement--modified compressive-shearing test by finite element analysis.

Failure of ultra-high molecular weight polyethylene components after total disc replacements in the lumbar spine has been reported in several retrieval studies, but immediate biomechanical evidence for those mechanical failures remained unclear. Current study aimed to investigate the failure mechanisms of commercial lumbar disc prostheses and to enhance the biomechanical performances of polyethylene components by modifying the articulating surface into a convex geometry. Modified compressive-shearing tests were utilized in finite element analyses for comparing the contact, tensile, and shearing stresses on two commercial disc prostheses and on a concave polyethylene design. The influence of radial clearance on stress distributions and prosthetic stability were considered. The modified compressive-shearing test revealed the possible mechanisms for transverse and radial cracks of polyethylene components, and would be helpful in observing the mechanical risks in the early design stage. Additionally, the concave polyethylene component exhibited lower contact and shearing stresses and more acceptable implant stability when compared with the convex polyethylene design through all radial clearances. Use of a concave polyethylene component in lumbar disc replacements decreased the risk of transverse and radial cracks, and also helped to maintain adequate stability. This design concept should be considered in lumbar disc implant designs in the future.

Biomechanical evaluation of proximal tibial behavior following unicondylar knee arthroplasty: modified resected surface with corresponding surgical technique.

Persistent pain and periprosthetic fracture of the proximal tibia are troublesome complications in modern unicondylar knee arthroplasty (UKA). Surgical errors and acute corners on the resected surface can place excessive strains on the bone, leading to bone degeneration. This study attempted to lower strains by altering the orthogonal geometry and avoiding extended vertical saw cuts. Finite element models were utilized to predict biomechanical behavior and were subsequently compared against experimental data. On the resected surface of the extended saw cut model, the greatest strains showed a 50% increase over a standard implant; conversely, the strains decreased by 40% for the radial-corner shaped model. For all UKA models, the peak strains below the resection level increased by 40% relative to an intact tibia. There was no significant difference among the implanted models. This study demonstrated that a large increase in strains arises on the tibial plateau to resist a cantilever-like bending moment following UKA. Surgical errors generally weaken the tibial support and increase the risk of fractures. This study provides guidance on altering the orthogonal geometry into a radial-shape to reduce strains and avoid degenerative remodeling. Furthermore, it could be expected that predrilling a posteriorly sloped tunnel through the tibia prior to cutting could achieve greater accuracy in surgical preparations.

Influence of post-cam design of posterior stabilized knee prosthesis on tibiofemoral motion during high knee flexion.

BACKGROUNDS: The post-cam design of contemporary posterior stabilized knee prosthesis can be categorized into flat-on-flat or curve-on-curve contact surfaces. The curve-on-curve design has been demonstrated its advantage of reducing stress concentration when the knee sustained an anteroposterior force with tibial rotation. How the post-cam design affects knee kinematics is still unknown, particularly, to compare the difference between the two design features. Analyzing knee kinematics of posterior stabilized knee prosthesis with various post-cam designs should provide certain instructions to the modification of prosthesis design. METHODS: A dynamic knee model was utilized to investigate tibiofemoral motion of various post-cam designs during high knee flexion. Two posterior stabilized knee models were constructed with flat-on-flat and curve-on-curve contact surfaces of post-cam. Dynamic data of axial tibial rotation and femoral translation were measured from full-extension to 135°. FINDINGS: Internal tibial rotation increased with knee flexion in both designs. Before post-cam engagement, the magnitude of internal tibial rotation was close in the two designs. However, tibial rotation angle decreased beyond femoral cam engaged with tibial post. The rate of reduction of tibial rotation was relatively lower in the curve-on-curve design. From post-cam engagement to extreme flexion, the curve-on-curve design had greater internal tibial rotation. INTERPRETATION: Motion constraint was generated by medial impingement of femoral cam on tibial post. It would interfere with the axial motion of the femur relative to the tibia, resulting in decrease of internal tibial rotation. Elimination of rotational constraint should be necessary for achieving better tibial rotation during high knee flexion.

Hybrid assembly of metal head and femoral stem from different manufacturers during isolated acetabular revision.

During revision of a failed acetabular component with well-fixed antiquated femoral stem, the same variant of metal head is usually not available in the inventory production. We gathered the stems retrieved from revision surgeries and tried to reassemble with the commercially available femoral heads. This study reports the results of suitable hybrid assembly of metal heads with the most common antiquated femoral stems from different manufacturers. Three hybrid combinations were determined and pull-off strengths of these hybrid combinations were performed through in vitro studies to evaluate the quality of the taper locks.

Design and test of hip stem for medullary revascularization.

This study aims at developing a femoral stem which maintains appropriate mechanical properties for clinical use and provides enough medullary space for revascularization. Hollow and drilled stems were designed to gain sufficient medullary space. Three-dimensional finite element models of the hollow stems were built including the hollow parameters of different wall thicknesses, bore depths and transverse hole patterns. The effects of these parameters on stress under load bearing were investigated. The results indicated that the effects of thickness on the stem depended on their location along the stem. The hollow depth should be designed as deep as possible to ensure a smooth change of cross-sectional area and avoid unexpected stress concentrations. The transverse hole pattern must be arranged very carefully to avoid high stress concentrations. Then the proper parameters of hollow stem were determined. Through the above processes, the appropriate shape of femoral stem for medullary revascularization was determined as a hollow structure with gradually changing thickness and a distinct hole pattern. Fatigue tests were conducted on three prototypes, which all three prototypes passed. The finite element models and design processes were therefore proved useful for our purpose.

Particle size and morphology of UHMWPE wear debris in failed total knee arthroplasties--a comparison between mobile bearing and fixed bearing knees.

Osteolysis induced by ultrahigh molecular weight polyethylene wear debris has been recognized as the major cause of long-term failure in total joint arthroplasties. In a previous study, the prevalence of intraoperatively identified osteolysis during primary revision surgery was much higher in mobile bearing knee replacements (47%) than in fixed bearing knee replacements (13%). We postulated that mobile bearing knee implants tend to produce smaller sized particles. In our current study, we compared the particle size and morphology of polyethylene wear debris between failed mobile bearing and fixed bearing knees. Tissue specimens from interfacial and lytic regions were extracted during revision surgery of 10 mobile bearing knees (all of the low contact stress (LCS) design) and 17 fixed bearing knees (10 of the porous-coated anatomic (PCA) and 7 of the Miller/Galante design). Polyethylene particles were isolated from the tissue specimens and examined using both scanning electron microscopy and light-scattering analyses. The LCS mobile bearing knees produced smaller particulate debris (mean equivalent spherical diameter: 0.58 microm in LCS, 1.17 microm in PCA and 5.23 microm in M/G) and more granular debris (mean value: 93% in LCS, 77% in PCA and 15% in M/G).