Nikolai Steklov

The Chitranjan Ranawat Award: in vivo knee forces after total knee arthroplasty.

Tibial forces were measured in vivo during the first year after total knee arthroplasty in a 66 kg, 80-year-old man. Forces were measured during activities of daily living, rehabilitation, and exercise. Peak tibial forces recorded during walking increased up to 12 months postoperatively (2.8 times body weight). Tibial forces correlated with increasing speed during treadmill walking. Rising from a chair generated peak forces of 2.6 times body weight. Stair descent generated higher peak forces than stair ascent (3.3 versus 2.9 times body weight, respectively). Exercising on a stair-climbing machine generated forces close to two times body weight whereas stationary bicycling generated even lower forces, near one times body weight. In general, the tibial forces recorded during walking and stair climbing were lower than most predicted values. These measurements can be used to validate in vitro and mathematical models of the knee. This should lead to refined surgical techniques and to enhanced prosthetic designs that will improve patient function, patient quality of life, and longevity of total knee arthroplasty implants.

In Vivo Contact Stresses during Activities of Daily Living after Knee Arthroplasty

We implanted an electronic knee prosthesis to measure tibial forces in vivo during activities of daily living after total knee arthroplasty. We used tibial forces and knee kinematic data collected in vivo to calculate contact stresses using finite element analysis. The polyethylene insert was modeled as an elastoplastic material, and predicted contact stresses were validated using pressure sensitive sensors. Peak contact stresses generated during walking were similar but about 18% lower than those calculated for International Standards Organization (ISO)‐recommended wear simulation conditions. Stair climbing generated higher contact stresses (32 MPa) than walking (26 MPa). However, both high flexion activities (lunge and kneel) generated even higher contact stresses, with the lunge activity generating the highest stresses (56 MPa). The activities that generated high contact stresses also resulted in high equivalent plastic strain. However, the lunge activity generated dramatically higher plastic equivalent strain than the other activities. In vivo measurement of kinematics, forces, and contact stresses may be used to develop more clinically relevant wear simulator protocols. Contact stresses generated during high flexion activities were substantially higher and were largely due to the reduced contact area in deep flexion rather than due to an increase in contact forces. Our results support the use of “high flexion” designs that improve contact conditions and preserve contact area at high flexion angles.

Effect of tibial component varus on wear in total knee arthroplasty

INTRODUCTION Malalignment can result in poor clinical outcomes and increased wear. However, component malalignment can occur even when overall limb mechanical axis is within the normal anatomic range. We studied the effect of component malalignment in the presence of acceptable knee alignment in knee arthroplasty. METHODS Sixteen tibial inserts retrieved at revision surgery were laser-mapped to measure wear. Average implantation duration was 7.7 years (range, 1 to 13). Early (postprimary) and final (prerevision) radiographs were analyzed for overall alignment (limb, femoral and tibial components) and osteolysis. RESULTS The tibial components were initially aligned in a mean of 1.3 ± 1.7° varus (range, -1.5 to 4.5°), which increased to 3.2 ± 2.9° (range, -2.0 to 8.0°) at the time of revision (p=0.05). Tibial components initially placed in greater than 3° varus were associated with almost twice the volumetric penetration rate. Anatomic knee angles were 5.4 ± 0.9° valgus (range, 4.0 to 7.0°) in the post-primary radiographs and decreased in prerevision radiographs to 3.8 ± 2.6° (range, -1.0 to 7.5°), (p=0.04). DISCUSSION Tibial varus was associated with increased medial compartment wear and total wear, thus affecting osteolysis in addition to local destruction of the bearing surface. Varus malalignment as low as 3° may result in accelerated wear, even if overall limb alignment is nearly ideal. These results indicate that tibial component alignment is an important factor associated with tibial tray subsidence and polyethylene wear even when limb alignment is neutral.

Anterior cruciate ligament changes in the human knee joint in aging and osteoarthritis.

OBJECTIVE The development and patterns of spontaneous age-related changes in the anterior cruciate ligament (ACL) and their relationship to articular cartilage degeneration are not well characterized. This study was undertaken to investigate the types and temporal sequence of age-related ACL changes and to determine their correlation with cartilage lesion patterns at all stages of osteoarthritis (OA) development in human knee joints without prior joint trauma. METHODS Human knee joints (n = 120 from 65 donors ages 23-92) were obtained at autopsy, and ACLs and cartilage were graded macroscopically and histologically. Inflammation surrounding the ACL was assessed separately. RESULTS Histologic ACL substance scores and ligament sheath inflammation scores increased with age. Collagen fiber disorganization was the earliest and most prevalent change. The severity of mucoid degeneration and chondroid metaplasia in the ACL increased with the development of cartilage lesions. A correlation between ACL degeneration and cartilage degeneration was observed, especially in the medial compartment of the knee joint. CONCLUSION Our findings indicate that ACL degeneration is highly prevalent in knees with cartilage defects and may even precede cartilage changes. Hence, ACL deficiencies may not only be important in posttraumatic OA, but may also be a feature associated with knee OA pathogenesis in general.

Vimentin contributes to changes in chondrocyte stiffness in osteoarthritis.

Actin and tubulin cytoskeletal components are studied extensively in chondrocytes, but less is known about vimentin intermediate filaments. In other cell types, vimentin is a determinant of cell stiffness and disruption of vimentin networks weakens the mechanical integrity of cells. Changes in vimentin organization correlate with osteoarthritis progression, but the functional consequences of these changes remain undetermined in chondrocytes. The objective of this study was to compare the contribution of vimentin to the mechanical stiffness of primary human chondrocytes isolated from normal versus osteoarthritic cartilage. Chondrocytes were embedded in alginate and vimentin networks disrupted with acrylamide. Constructs were imaged while subjected to 20% nominal strain on a confocal microscope stage, and the aspect ratios of approximately 1,900 cells were measured. Cytosolic stiffness was estimated with a finite element model, and live‐cell imaging of GFP‐vimentin was used to further analyze the nature of vimentin disruption. Vimentin in normal chondrocytes formed an inner cage‐like network that was substantially stiffer than the rest of the cytosol and contributed significantly to overall cellular stiffness. Disruption of vimentin reduced stiffness approximately 2.8‐fold in normal chondrocytes. In contrast, osteoarthritic chondrocytes were less stiff and less affected by vimentin disruption. This 3D experimental system revealed contributions of vimentin to chondrocyte stiffness previously not apparent, and correlated changes in vimentin‐based chondrocyte stiffness with osteoarthritis.

Knee joint forces: prediction, measurement, and significance

Knee forces are highly significant in osteoarthritis and in the survival and function of knee arthroplasty. A large number of studies have attempted to estimate forces around the knee during various activities. Several approaches have been used to relate knee kinematics and external forces to internal joint contact forces, the most popular being inverse dynamics, forward dynamics, and static body analyses. Knee forces have also been measured in vivo after knee arthroplasty, which serves as valuable validation of computational predictions. This review summarizes the results of published studies that measured knee forces for various activities. The efficacy of various methods to alter knee force distribution, such as gait modification, orthotics, walking aids, and custom treadmills are analyzed. Current gaps in our knowledge are identified and directions for future research in this area are outlined.

Wear of Polyethylene Against Oxidized Zirconium Femoral Components: Effect of Aggressive Kinematic Conditions and Malalignment in Total Knee Arthroplasty

Metallic femoral components with ceramic articulating surfaces can substantially lower polyethylene (PE) wear during walking activities under conditions of normal knee alignment. It is unknown whether these types of components can maintain low wear rates under conditions of knee malalignment and the harsher kinematics associated with younger, athletically active patients. Wear was measured in non-cross-linked, ethylene oxide-sterilized PE inserts against oxidized zirconium or cobalt-chrome femoral components in a knee wear simulator. The vertical load was modified to replicate knee varus malalignment of 3°, and the range of tibial rotation was increased to 20°. Mean gravimetric and volumetric wear rate over 5 million cycles was 55% lower in the oxidized zirconium group. An oxidized zirconium femoral component can significantly reduce PE wear under simulated conditions of athletically active patients with modestly malaligned total knee arthroplasty prostheses.

Predicting the effect of tray malalignment on risk for bone damage and implant subsidence after total knee arthroplasty

Tibial tray malalignment has been associated with increased subsidence and failure. We constructed a finite element model of knee arthroplasty to determine the biomechanical factors involved in increasing the risk of subsidence with malalignment. Four fresh‐frozen human knees were implanted with a tibial tray and subjected to forces representative of walking for up to 100,000 cycles. Cyclic displacement was measured between the tray and proximal tibia. The vertical load was shifted medially to generate a load distribution ratio of 55:45 (medial/lateral) to represent neutral alignment or 75:25 to represent varus alignment. Subjected specific geometry and material properties were obtained from qCT scans of tibia to construct a finite element model. The tray was subjected to a single load cycle representing experimental conditions. Tray displacement computed by the model matched that measured experimentally. Forces representing varus tray alignment generated greater strains in the proximal tibia and a greater volume of bone was subjected to strains higher than the fatigue threshold. Local compressive strains directly correlated with experimental subsidence and failure. Our results indicate that failure after tray malalignment is likely due to fatigue damage to the proximal tibia rather than shear across the implant–bone interface or failure of the cement mantle.

Effect of Osteochondral Graft Insertion Forces on Chondrocyte Viability

Background Because chondrocytes are responsible for articular cartilage matrix synthesis and maintenance, reduced chondrocyte viability could compromise graft survival, healing, and clinical outcome. Hypothesis Typical forces used in osteochondral grafting reduce the viability of the chondrocytes in the graft. Study Design: Controlled laboratory study. Methods Osteochondral grafting was performed in 4 fresh-frozen cadaver knees (n = 16 per knee). Impact force was measured during extrusion of the donor graft from the harvester into the recipient site, seating the graft flush with the articular surface of the surrounding cartilage using a tamp, and recessing the graft surface below the recipient articular surface. The magnitudes of forces measured during cadaver surgery (200,400, and 800 N) were reproduced using a drop-tower apparatus on 80 fresh osteochondral grafts harvested from knee blocks provided by tissue banks. Cell viability and glycosaminoglycan release in media were measured at 48 hours after injury. Results Forces were relatively low (range, 124-356 N) during graft extrusion from the harvester into the recipient defect or during flush seating (range, 191-418 N) of the graft. Attempts to recess the graft generated significantly greater force (range, 147-685- P < .01). When the donor graft length was 2 mm longer than the depth of the recipient hole, the mean impact force generated was even higher (range, 240-1114 N) than the force seen in a donor graft of equal length. No reduction in viability was seen at 200-N and 400-N impacts. However, a significant decrease in chondrocyte viability was seen in the group impacted with 800 N (only 50% of cells were viable, compared with 91 % in the sham group; P < .01). Glycosaminoglycan levels in culture media did not correlate significantly with insertion force. Conclusion Typical graft insertion forces did not significantly reduce chondrocyte viability. However, increased graft length relative to the depth of the recipient hole and attempts to recess the graft generated higher forces, which reduced chondrocyte viability. Clinical Relevance Any theoretical benefits of cancellous bone compaction that may occur in grafts that are recessed or are longer than the recipient holes must be balanced against the potential reduction in chondrocyte viability.

Anti‐gravity treadmills are effective in reducing knee forces

Lower body positive pressure (LBPP) treadmills permit significant unweighting of patients and have the potential to enhance recovery following lower limb surgery. We determined the efficacy of an LBPP treadmill in reducing knee forces in vivo. Subjects, implanted with custom electronic tibial prostheses to measure forces in the knee, were tested on a treadmill housed within a LBPP chamber. Tibiofemoral forces were monitored at treadmill speeds from 1.5 mph (0.67 m/s) to 4.5 mph (2.01 m/s), treadmill incline from −10° to +10°, and four treadmill chamber pressure settings adjusted to decrease net treadmill reaction force from 100% to 25% of the subjects body weight (BW). The peak axial tibiofemoral force ranged from 5.1 times BW at a treadmill speed of 4.5 mph (2.01 m/s) and a pressure setting of 100% BW to 0.8 times BW at 1.5 mph (0.67 m/s) and a pressure setting of 25% BW. Peak knee forces were significantly correlated with walking speed and treadmill reaction force (R2 = 0.77, p = 0.04). The LBPP treadmill might be an effective tool in the rehabilitation of patients following lower‐extremity surgery. The strong correlation between tibiofemoral force and walking speed and treadmill reaction forces allows for more precisely achieving the target knee forces desired during early rehabilitation.