Fixation of the Quatroloc femoral component: a biomechanical and clinical study.

The current study was designed to test the hypothesis that press-fit femoral components with proximal press-fit and distal mechanical interlock can achieve fixation sufficient to allow bone ingrowth in osteoporotic and in normal bone. The addition of steps along the tapered distal stem improved fixation in osteoporotic bone enough to reduce micromotion to less than 20 microm in response to physiologic axial and torsional load. The clinical portion of the study included 226 consecutive hips (223 patients) with 2- to 4-year clinical results after total hip arthroplasty with a rectangular femoral component using proximal porous coating and distal mechanical interlock. Patient age ranged from 36 to 92 years. At 2 years postoperative, 4% of the patients with Type A (normal) femoral bone, 3% with Type B (intermediate) bone, and no patients with Type C (osteoporotic) bone had thigh pain. No clinical cases of loosening have occurred in normal or osteoporotic femurs.

Knee stability in meniscal bearing total knee arthroplasty.

The effect of a meniscal bearing on knee laxity in anterior cruciate ligament-sacrificing total knee arthroplasty was evaluated in 7 cadaver knees using a knee testing device that measured knee flexion angle as well as laxity to medial-lateral, anterior-posterior [AP], and rotational loads. A standard fixed tibial component and mobile tibial components (AP sliding, rotationally sliding, and AP and rotationally sliding) were used to evaluate AP, rotational, and varus-valgus stability and maximal flexion and extension with the neutrally positioned and malrotated tibial tray. The AP movable components increased AP laxity, and the fixed component decreased rotational laxity significantly when compared with the normal knees. The rotationally movable components did not change knee laxities significantly even when the tibial tray was malrotated. No significant difference among the components was detected when the maximal flexion and extension angles were compared in the neutrally positioned tibial tray. Malrotation of the tibial tray decreased the maximal extension angle in the fixed component. This study showed that the rotationally movable component can achieve near-normal laxity regardless of tibial tray rotation, but AP mobility of the bearing produces AP laxity that could lead to implant failure.

Contact stress analysis in meniscal bearing total knee arthroplasty.

The effect of a mobile meniscal bearing on tibiofemoral contact stress was tested with a standard fixed tibial component and with movable tibial components (anteroposterior sliding, rotationally sliding, and anteroposterior and rotationally sliding). A digital electronic sensor was used to detect tibiofemoral contact location in five cadaver knees, then the location was reproduced while peak and mean stresses were measured under compressive load at 0 degrees, 30 degrees, 60 degrees, and 90 degrees of flexion. Stresses were measured when the tibial component was normally aligned and at 15 degrees internal and 15 degrees external rotation. To evaluate the effect of excessive overhang of the polyethylene articular surface, undersurface stress of the rotationally sliding component was also measured with a 30 degrees and a 45 degrees malrotated tibial tray. Uppersurface stresses of the fixed-bearing components were significantly higher at full extension than those recorded in components with rotational mobility. Undersurface stresses were always lower than uppersurface stresses, but correlated with uppersurface stresses. Undersurface stresses of the rotationally sliding component gradually increased as the malrotation angle of the tray increased. A mobile meniscal bearing surface appears to offer an advantage over a standard fixed component when rotational malalignment of the tibial component occurs. However, with severe rotational malalignment, edge contact markedly increases undersurface stresses, which could cause deformity and subluxation.

Effect of partial release of the posterior cruciate ligament in total knee arthroplasty.

Appropriate tension of the posterior cruciate ligament, which often is tight in deep flexion, is difficult to achieve after posterior cruciate ligament retaining total knee arthroplasty. Kinematics and maximum flexion after partial release of the posterior cruciate ligament were evaluated in this study. A partial release improved the maximum flexion angle and maintained anteroposterior stability without causing undesirable changes in kinematics, whereas full resection of the posterior cruciate ligament caused unfavorable anteroposterior instability. Partial posterior cruciate ligament release eliminated excessive rollback movement caused by a tight posterior cruciate ligament and also shifted the point of articular surface contact anteriorly. These results indicate that partial release of the posterior cruciate ligament may improve knee function in patients with a tight posterior cruciate ligament after total knee arthroplasty.

Knee kinematics of posterior cruciate ligament sacrificed total knee arthroplasty.

Two different posterior stabilizing mechanisms in posterior cruciate ligament sacrificing total knee arthroplasty were evaluated to test knee kinematics, tibiofemoral contact location, and the quadriceps force needed to extend the knee. A posterior stabilizing post and a deeply dished polyethylene with a high anterior lip (anterior buildup stabilizing tibial articular surface) were tested in four anatomic specimen knees with a knee kinematics testing device. Both posterior stabilizing mechanisms partially restored anteroposterior stability, but the knees were still more lax than were the normal knees in the anteroposterior direction. The anterior buildup stabilizing tibial articular surface decreased rotational laxity in early flexion. Tibiofemoral contact location in the knees with the anterior buildup stabilizing tibial articular surface did not move posteriorly as the knee flexed, and the knees with this articular surface required significantly more quadriceps force to extend the knee than did the knees with a stabilizing post.

Fixation of ultrahigh-molecular-weight polyethylene liners to metal-backed acetabular cups.

Locking mechanisms and metal-liner interface surfaces of six modular acetabular systems were evaluated to determine their effect on micromotion and backside wear of the polyethylene liner. Rotational and axial motion between the metal shell and polyethylene liner was measured in the Duraloc (DePuy, Warsaw, IN), Harris-Galante (Zimmer, Warsaw, IN), Impact (Biomet, Warsaw, IN), Lip Loc (Biomet), Precision Osteoloc (Howmedica, Rutherford, NJ), and Reflection (Smith & Nephew Orthopaedics, Memphis, TN) designs at the start of each test, and at 1 million, 5 million, and 10 million cycles. At 10 million cycles, the Lip Loc and Reflection cups had significantly lower rim micromotion than the Duraloc and Harris-Galante cups (F < .0010). The Impact, Precision Osteoloc, and Reflection cups had significantly lower rim subsidence than the Harris-Galante cup (F < .0025). The Harris-Galante cup had significantly greater rotational micromotion than the Lip Loc cup (F < .0074), and had significantly greater interface slippage than the Impact and Reflection cups (F < .0070). The Lip Loc produced significantly lower dome micromotion than the Harris-Galante (F < .0300). The Lip Loc and Reflection cups had significantly less backside wear than the Duraloc and Harris-Galante cups (P < .0001), the Impact cup (P < .0243), and the Precision Osteoloc (P < .0027) cup.

Rotational stability of noncemented total hip femoral components.

Achieving torsional fixation of the femoral component in total hip arthroplasty is an important factor in success of the implant, so design features that improve fixation are likely to improve clinical results. Four femoral stem designs that allow different levels of femoral neck resection and use different distal fixation techniques were mechanically tested in cadaveric femurs to determine resistance to torsional loads. Five specimens for each stem design were implanted according to the standard procedure, and each was axially and torsionally loaded in a servohydraulic testing machine. Rotational interface micromotion and interface slippage were measured at the bone-implant interface. Decreased micromotion and interface slippage were associated with a distal scratch fit and neck retaining design, and the combination of distal scratch fit and neck retention had the least micromotion and smallest standard deviation of the four designs.

Effect of medial displacement of the tibial tubercle on patellar position after rotational malposition of the femoral component in total knee arthroplasty.

A large Q angle induced by technical error such as an internally rotated femoral component causes patellar failure after total knee arthroplasty. The effect of medial displacement of the tibial tubercle to decrease the Q angle for patellar tracking was studied by evaluating the patellar position relative to the patellar groove on the femoral component in cadaver specimens. A 5 degrees internally rotated femoral component caused the patella to shift medially about 5 mm, and also caused the tibia to rotate internally about 3 degrees at full extension. With a 5 degrees externally rotated femoral component, normal patellar tracking occurred. The distance of medial displacement was determined so that the patellar tendon was parallel to the longitudinal axis of the tibia at full extension. This allowed the quadriceps tendon, the patella, and the patellar tendon to form a straight line. The average distance of medial transposition of the tibial tubercle was 9.32 mm. Medialization of the tibial tubercle caused the patella to shift about 2 mm medially from the patellar groove. The transfer also caused an external rotation of the tibia (2 degrees-5 degrees). Medial transfer of the tibial tubercle changes patellar kinematics and corrects the tendency toward lateral patellar dislocation caused by internally rotating the femoral component; however, it also creates minor patellar and tibial kinematic changes that may have a clinical effect.

Effect of neck resection on torsional stability of cementless total hip replacement.

Loosening of the femoral component in total hip arthroplasty commonly results from inadequate resistance to torsional loads. We evaluated 20 adult human cadaver femora to determine the effect of different neck-resection levels on torsional resistance of the femoral component. All specimens were prepared for fixation with the Impact modular total hip replacement. Each femoral diaphysis was overreamed 2 mm to achieve only proximal fixation. The specimens were then divided into groups of five and implants were inserted with the precision press-fit technique. Each specimen was loaded in an Instron stress-testing device. A linearly variable differential transducer was then attached to the specimen to measure micromotion at the medial interface between the implant and bone. Each specimen was loaded until failure occurred. When all of the neck was preserved, torsional load to failure was significantly better than in the 50%, 15%, and 0% neck-preservation specimens. At a 20 N-m torsional load, the 100% and 50% neck preservation specimens had similar micromotion, but the 15% and 0% specimens had gross motion and a large standard deviation at this load level. Without distal fixation, the femoral component is highly dependent on proximal geometry for resistance to torsional loading. Preserving the femoral neck provides an effective means of resistance. Maintaining the entire femoral neck most effectively reduces miromotion at low loads, but maintaining the midshaft area of the femoral neck appears to most effectively control micromotion at higher torsional loads. Resection below the midshaft of the neck markedly decreases the torsional load-bearing capacity of the proximal femur.

Femoral rotational alignment, based on the anteroposterior axis, in total knee arthroplasty in a valgus knee. A technical note.

The landmarks used to achieve correct rotational alignment of the femoral component in total knee arthroplasty may be indistinguishable or unreliable in the distal architecture of a valgus knee. Five observers identified the anteroposterior axis, the posterior condylar axis, and the transepicondylar axis in thirty cadaveric femora to determine the reliability of the use of each axis in the operative setting. In addition, radiographs were made of the distal aspect of each femur, the axes were constructed, and the angles were measured and compared with the visual measurements made by the observers. A line drawn perpendicular to the anteroposterior axis consistently approximated 4 degrees of external rotation relative to the posterior condylar surfaces. The transepicondylar axis was more difficult to define and was not as accurate. The radiographic results were similar to the visual results, but the standard deviations for the former were less than those for the latter. The anteroposterior axis appears to be a reliable landmark for rotational alignment of the femoral component in a valgus knee.

Effect of rotational malposition of the femoral component on knee stability kinematics after total knee arthroplasty.

Excessive external rotation of the femoral component can cause an abnormally tight popliteus tendon complex, which induces loss of rotational laxity of the knee in the late phase of knee flexion after total knee arthroplasty. This study evaluated the effect of popliteus tendon release on rotational and varus-valgus laxity of implanted knees with an excessively externally rotated femoral component. Rotational and varus-valgus laxity was measured with a knee kinematics testing device before and after total knee arthroplasty. External rotational positions of the femoral component of 5 degrees and 8 degrees were compared, and the effects of popliteus tendon release on rotational and varus-valgus laxity were evaluated. To further investigate this question, the effect of a conforming articular design was compared with that of a flat tibial surface. External rotational position of 5 degrees did not change rotational or varus-valgus laxity of the knee. With an 8 degrees external rotational position, however, external rotational laxity significantly decreased in knees with a conforming surface at angles of 30 degrees, 45 degrees, 60 degrees, and 90 degrees. After popliteus tendon release, external rotational laxity significantly improved at 90 degrees flexion and was identical to that of the normal knee. Internal rotational range was similar before and after popliteus tendon release. Popliteus tendon release did not affect the varus-valgus laxity (stability) with either articular surface.

Reaming technique of the femoral diaphysis in cementless total hip arthroplasty.

Distal fixation enhances initial torsional stability of cementless femoral components in primary and revision total hip arthroplasty. Surgical technique affects the quality of distal fixation, and too aggressive a technique can cause femoral fracture during insertion of the stem. Thirty-five adult human cadaveric femoral specimens were implanted with either a standard or long femoral stem. Control specimens were reamed 0.5 mm less than the diameter of the distal cylindrical portion of the stem and were broached line to line proximally. The proximal femur was removed from the remaining specimens to allow assessment of distal fixation. The controls outperformed the distal-only fixation groups in all testing modes, illustrating that proximal and distal fixation work in concert to resist torsional load. The average failure torque in underreamed specimens with only distal fixation was 23.6 Nm for standard length stem specimens and 41.3 Nm for long stem specimens, whereas the average failure torque for specimens with only distal fixation prepared by a line-to-line reaming technique was only 6 Nm. The estimated length of tight distal fit should be 10 to 40 mm to obtain sufficient initial torsional stability of the stem and still avoid intraoperative femoral fracture.

Patellar tracking measurement in the normal knee.

Eleven fresh frozen cadaveric knee specimens were mounted in a knee kinematics test device, and normal patellar movements were evaluated with use of an external device for direct measurement of patellar movements. The effects of four different measurement conditions were assessed through alteration of one condition and determination of its effect on patellar kinematics with the use of six specimens. The four conditions included (a) change of the measuring axis from an axis parallel to the central axis of the femur (femoral axis) to one parallel to the central axis of the tibia (tibial axis), (b) rotation of the femoral axis internally 6 degrees, (c) change of the direction of the quadriceps force from parallel to the mechanical line of the lower extremity to a direction parallel to the femoral shaft, and (d) increase of the magnitude of the quadriceps force from 111 to 500 N. During knee flexion, the patella shifted laterally after a slight initial medial shift, tilted laterally from midflexion to 90 degrees, and gradually rotated medially. The patellar shift relative to the tibial axis appeared to be more medial than the shift measured relative to the femoral axis; the discrepancy was caused by the valgus position of the tibia relative to the femur. Changing the rotational angle of the femoral axis artificially changed the patellar position. Varying the direction of the quadriceps within the narrow range and increasing the quadriceps force did not affect patellar movements.(ABSTRACT TRUNCATED AT 250 WORDS)

Simulated knee wear with cobalt chromium and oxidized zirconium knee femoral components.

A knee simulator that mimics the plowing/rolling wear mechanisms of the knee was used to compare wear properties of cobalt chromium and oxidized zirconium femoral components. The simulator flexes and extends the knee so that the femoral components travels from 0 degrees to 30 degrees while applying axial loads from 130 to 1300 lb. Three oxidized zirconium and 3 cobalt chromium femoral components were tested with 10-mm tibial polyethylene components. The oxidized zirconium femoral components caused significantly less ultra high molecular weight polyethylene wear than cobalt chromium femoral components. Tibial inserts that were articulated against the cobalt chromium components had evidence of scratching, burnishing, and delamination, but none of the surfaces that were articulated against oxidized zirconium components had evidence of delamination. Cobalt chromium surface roughness significantly increased during the 2,000,000 cycle test, but oxidized zirconium surface roughness was not affected. Polyethylene wear was correlated to a significant degree with the surface roughness of the femoral components. The improved wear characteristics of the ceramic articular surfaces can be explained by the wettability of the ceramic surface, which minimized adhesive wear, and the resistance of the hard, ceramic surface to roughening.

Patellar tracking after total knee arthroplasty. The effect of tibial tray malrotation and articular surface configuration.

The effect of total knee arthroplasty (TKA) with neutrally aligned and malrotated tibial trays was studied in five fresh anatomic specimen knees. Patellar shift, tilt, and rotation, and the rotational position of the tibia were measured in normal knees and after TKA with the Ortholoc Modular knee system. Both semiconstrained and unconstrained articular surfaces were assessed in the neutral position and at anatomic, 15 degrees internal, and 15 degrees external rotation of the tibial tray. After TKA, the patellae shifted slightly medially in the early phase of knee flexion because the anterior lateral flange of the femoral component was longer than the lateral trochlea of the femur and because the tibia rotated internally. The raised lateral flange on the femoral component tilted the patella medially at full extension after TKA. The semiconstrained tray allowed minimal tibial rotation because of its articular configuration. As much as 15 degrees malrotation of the unconstrained tibial tray did not affect patellar tracking. The semiconstrained tibial tray in the neutral position had almost the same patellar tracking as the unconstrained tray, but at 15 degrees external rotation, the semiconstrained tray internally rotated the tibia, leading to medial shift of the patella. Although 15 degrees internal rotation caused external rotation of the tibia, the patella did not shift as much laterally, despite the increase in the Q angle.

Fixation of the modular total hip femoral component in cementless total hip arthroplasty.

Cementless femoral component fixation with a modular design that achieved tight fit in the proximal and distal femur was evaluated. The base of the femoral neck made the most important contribution, but diaphyseal bony interlock also contributed significantly to immediate torsional fixation. Fluted surface and slotted distal design produced fixation that was superior to fully porous-coated and cemented cylindrical stems. Early cyclical loading improved fixation, and under moderate torsional loads (up to 20 N-m), no measurable interface slippage occurred with the modular implant.

Effects of femoral component material properties on cementless fixation in total hip arthroplasty. A comparison study between carbon composite, titanium alloy, and stainless steel.

Carbon-fiber-reinforced-carbon composite material is an attractive implant material because its modulus of elasticity can be made similar to that of cortical bone. This study investigated the effect of femoral prosthesis elastic modulus on cementless implant fixation. Distal, as well as proximal, relative micromovements between implant and bone were measured in two testing protocols (axial-load and torsional-load), comparing identically shaped carbon composite (modulus of elasticity = 18.6 GPa), Ti6Al4V (100 GPa), and 630 stainless steel (200 GPa) prostheses. In the axial-load test, proximal mediolateral micromotions were significantly larger in the flexible composite stem than in the two metals. In the torsional-load test, rotational micromotions and "slop" displacements in the flexible stem were significantly larger proximally and significantly smaller distally than in the two metals. While these results suggest that proximal stress transfer may be improved by a flexible stem, they raise the possibility of increased proximal micromotion, and suggest that improved proximal fixation may be necessary to achieve clinical success with flexible composite femoral components.

Torsional fixation of a modular femoral hip component.

Standard range of motion (S-ROM) modular total hip femoral stem has been designed to achieve precise fit in the diaphysis and metaphysis. This study compares torsional fixation of the S-ROM stem implanted with tight fixation of both the metaphyseal and diaphyseal components, tight metaphyseal and loose diaphyseal fixation. The mean load to failure was significantly greater in those implanted with tight fixation proximally and distally. Rotational micromotion was significantly decreased by fixation proximally, but permanent rotational displacement was improved primarily by distal fixation. Tight proximal and distal fixation were necessary to control both micromotion and permanent rotational displacement caused by torsional loading. Clinical success of this implant will depend on achieving tight proximal and distal fixation.

Laboratory evaluation of alignment and kinematics in a unicompartmental knee arthroplasty inserted with intramedullary instrumentation.

The purposes of this study were to evaluate the reliability of intramedullary (IM) instrumentation for unicompartmental total knee replacement and to assess the stability characteristics of the knee after implantation of a relatively unconstrained articular surface. Five adult, human cadaver lower extremities including hip, knee, and ankle were used to evaluate IM alignment. Five adult, fresh-frozen knee specimens were used to evaluate knee kinematics. Long anterioposterior roentgenograms were used to evaluate valgus angle and position of the center of the knee relative to the mechanical axis of the lower extremity. IM instrumentation returned the knee to normal alignment in all cases. The greatest valgus angle change was 3 degrees, and the position of the center of the knee relative to the mechanical axis was not significantly altered. Knee kinematics after unicompartmental knee replacement followed the predicted pattern of normal stability in extension and had slightly less varus-valgus laxity at 30 degrees (p less than 0.01), 45 degrees (p less than 0.01), and 60 degrees (p less than 0.05), and less anteroposterior displacement at 45 degrees (p less than 0.01) and 60 degrees (p less than 0.05). This study offers encouraging evidence that unicompartmental knee replacement with unconstrained components can restore normal knee kinematics, and that alignment can be restored with a high degree of accuracy with an intramedullary alignment system.