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Summary
Knee kinematics were analyzed in subjects with unicom- partmental knee arthroplasty (UKA) during deep knee bend (DKB) and stance phase of gait, using video fluo- roscopy.Femorotibial contact positions were determined using a computer-automated model-fitting technique.
During DKB, subjects with medial UKA experienced on average -0.8 mm of posterior femoral roll-back (PFR), those with lateral UKA -2.5 mm. During stance phase of gait, subjects with medial UKA experienced on average 0.8 mm of PFR, those with lateral UKA -0.4 mm. During DKB normal axial rotation was found for two thirds of subjects (average = 3.3° and 11.2°, respectively, for medial and lateral UKA) and during stance phase of gait for half of the subjects with medial UKA and one quarter with lat- eral UKA (average of 0.9° and -6.0°, respectively). On av- erage,subjects with UKA experienced kinematic patterns similar to those of the normal knee. The kinematic vari- ability for some subjects suggests progressive laxity of the anterior cruciate ligament, which confirms its role for maintaining satisfactory knee kinematics in UKA.
Introduction
Since the introduction of unicompartmental knee arthro- plasty (UKA), many improvements have been made in patient selection, preoperative planning, and surgical in- strumentation [1-3], while recently, due to the technical evolution in the field of minimally invasive surgery, a re- newed interest in UKA has been noted among knee sur- geons [4].
The main failure mode for UKA has been polyethylene wear [5, 6]. More recently, failures secondary to cata- strophic polyethylene wear have been observed,attributed to less conforming articular geometries,polyethylene ster- ilization methods, or disturbed knee kinematics. A better understanding of knee joint kinematics is important to ex- plain the premature polyethylene wear failures observed and serves as the purpose of this investigation.
To date, most experimental studies of knee kinemat- ics have involved cadaveric in vitro analyses or have not
tested the knee in a weight-bearing mode [7]. Previously, we determined the in vivo femorotibial and patello- femoral motions of various types of TKA using video flu- oroscopy, demonstrating numerous kinematic variances compared with the normal knee, such as paradoxical an- terior femoral translation and femoral condylar liftoff [8, 9]. The purpose of the present study was to analyze the kinematics of UKA in which the ACL was intact at the time of the operative procedure, under in vivo weight- bearing conditions, during a deep knee bend, and during stance phase of gait.
Material and Methods
All subjects evaluated received the same UKA (M/G Uni- compartmental Arthroplasty, Zimmer, Warsaw, IN, USA) and were operated on in the same department of ortho- pedic surgery in Marseille, France. For the deep knee bend (DKB) study, 20 knees were evaluated, 17 implanted with a medial UKA and three with a lateral UKA. For the gait analysis study, 19 knees were evaluated, 15 implanted with a medial UKA and four with a lateral UKA.All UKAs were judged as clinically successful (HSS scores >90), without substantial ligamentous laxity or pain, and the ACL was considered to be functionally present in all cas- es.
Under fluoroscopic surveillance, each subject was asked to perform successive weight-bearing deep knee bend maneuvers up to maximum flexion, while the knee kinematic patterns were assessed at full extension and at 15°,30°,45°,60°,75°,and 90° of knee flexion.Likewise,each subject was asked to perform the normal stance phase of gait, while the kinematics were analyzed at heel-strike, at 33% and 66% of stance phase, and at toe-off.
The contact position between the medial femoral condyle (medial UKA) or the lateral femoral condyle (lateral UKA) and the tibia was determined using a three- dimensional (3D) model-fitting technique [10]. The 3D computer-aided design (CAD) solid models of the femoral and tibial components were overlaid and fit onto the two-dimensional (2D) fluoroscopic perspective view images (
⊡ Fig. 22-1).
22 Kinematic Characteristics of the Unicompartmental Knee
J. N. Argenson, R. D. Komistek, D. A. Dennis
22
A contact position anterior to the midline of the tibia was denoted as positive, and a position posterior was denoted as negative. For rotation, the angle between the longitudi- nal axis of the femoral component on the coronal view and the fixed axis passing through the tibial component was measured either medially or laterally (
⊡ Fig. 22-2).
Error analyses for this 3D model-fitting technique have been conducted previously and demonstrated trans- lational errors less of than 0.5 mm, and rotational errors of less than 0.5° [10].
Results
Anteroposterior Translation
Medial UKA
On average, during DKB the femoral component moved 3.1 mm posteriorly from 0° to 45° and then 2.3 mm ante- riorly from 45° to 90° (
⊡ Fig. 22-3). Thus from 0° to 90° the femoral components moved an average of 0.8 mm poste- riorly. Eight knees (47 %) displayed anterior tibiofemoral contact at 0°. In the remaining nine knees the inital con- tact was more posterior.
At heel-strike, the average contact position for sub- jects with a medial UA was -0.2 mm (6.1 to -7.2), moving an average of 0.3 mm in the anterior direction to an aver- age contact position of 0.3 mm (6.6 to -7.2) at 33% of gait stance phase (
⊡ Fig. 22-4).The subjects with a medial UKA
remained in a similar position at 66% of stance phase with a contact position of 0.4 mm (7.7 to -6.0). From 66%
of stance phase to toe-off, these subjects experienced an average anterior motion, having a contact position of 0.6 mm (7.2 to -8.0) at toe-off. Eleven of the 15 subjects expe- rienced less than 2.0 mm of medial UKA motion (anteri- or or posterior),which is similar to the medial condyle for the normal knee during gait.
Lateral UKA
During DKB, two knees exhibited minimal anteroposte- rior motion of the femoral component from 0° to 90°,and one exhibited posterior motion of 7.6 mm. Between 0°
Chapter 22 · Kinematic Characteristics of the Unicompartmental Knee – J. N. Argenson et al. 149
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⊡ Fig. 22-1.Example of the 3D model-fitting process showing a fluoroscopic image (far left), 3D overlay (center left), pure sagittal view (center right), and top view (far right)
⊡ Fig. 22-2.Technique used to determine the axial rotation of the UKA AP Position (mm) [- Post., + Ant.]
10 5 0 -5 -10
-150 15 30 45 60 75 90
Flexion Angle (Degrees)
AP Position (mm) [- Post., + Ant.]
Flexion Angle (Degrees) 10.00
5.00 0.00 -5.00 -10.00 -15.00
HS 33 66 TO
⊡ Fig. 22-4.Average anteroposterior contact position, evaluated on the sagittal view, for subjects with a medial UKA, during stance phase of gait, from heel-strike (HS) to toe-off (TO)
⊡ Fig. 22-3.Average anteroposterior contact position, evaluated on the sagittal view, for subjects with a medial UKA, during deep knee bend
and 90°, all three knees exhibited both anterior and pos- terior motion at different points in the arc.
On average, during stance phase of gait, subjects with a lateral UA experienced -0.4 mm of posterior motion from heel-strike to toe-off (
⊡ Fig. 22-5).
At heel-strike,the average contact position for subjects with a lateral UA was -5.7 mm (-3.9 to -83.9),at 33% of stance phase the average was -6.4 mm (-5.7 to -7.6), at 66% of stance phase the average was -7.3 mm (-2.4 to -9.9), and at toe-off the average contact position was -6.1 mm (-4.3 to -8.0). On average, the greatest amount of posterior mo- tion occurred from heel-strike to 66% of stance phase (-1.6 mm),while an anterior slide of 1.2 mm occurred from 66% of stance phase to toe-off. Overall, all four subjects experienced less than 2.1 mm of motion, whether the motion occurred in the anterior or the posterior direction.
Axial Tibiofemoral Rotation
Medial UKA
On average, during DKB the 17 knees displayed 3.3° of in- ternal tibial rotation between the two components of the
prosthesis between 0° and 90° flexion (
⊡ Fig. 22-6). Three knees displayed external tibial rotation, two displayed negligible (<1.0°) rotation, and the remainder rotated in- ternally with a mean of 10.1° (range 1.2°-17.3°).
On average, during stance phase of gait, subjects with a medial UA experienced 0.94° of normal axial rotation from heel-strike to toe-off (
⊡ Fig. 22-7). Eight of 15 (53.3%) subjects with a medial UKA experienced normal axial rotation from heel-strike to toe-off (
⊡ Fig. 22-8).
150 III . Kinematics
22
AP Position (mm) [- Post., + Ant.]
Flexion Angle (Degrees) 10.00
5.00 0.00 -5.00 -10.00 -15.00
HS 33 66 TO
⊡ Fig. 22-5.Average anteroposterior contact position, evaluated on the sagittal view, for subjects with a lateral UKA, during stance phase of gait, from heel-strike (HS) to toe-off (TO)
Axial Rotation Angle (Deg)
2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2
-2.50 15 30 45 60 75 90
Knees Flexion Angle (Degrees)
⊡ Fig. 22-6.Average axial rotation pattern for subjects having a medial UKA, during deep knee bend
Rotation Angle (Degrees)
30.00 20.00 10.00 0.00 -10.00 -20.00 -30.00
HS 33 66 TO
Flexion Angle (Degrees)
⊡ Fig. 22-7.Average axial rotation pattern for subjects with a medial UKA, during stance phase of gait, from heel-strike (HS) to toe-off (TO)
⊡ Fig. 22-8.Top view of a subject with a medial UKA, depicting normal axial rotation during gait
Lateral UKA
During DKB, it was found that in two knees the tibia rotated internally with flexion, but in one knee the tibia rotated externally. As with AP motion, the direction of axial rotation in an individual knee changed from point to point over the range from 0° to 90°.
During stance phase of gait, and contrary to those with a medial UKA, subjects with a lateral UKA experi- enced on average -6.0° of opposite axial rotation from heel-strike to toe-off. Two of four subjects with a lateral UKA experienced greater than 10° of opposite axial rota- tion.
Discussion
Kinematic evaluations of the normal knee have demon- strated anterior femorotibial contact in full extension and 14.2 mm of posterior roll-back of the lateral femoral condyle (PFR) with progressive flexion, while during stance phase of gait minimal motion (1.0-2.0 mm) of the medial condyle and approximately 4.0 mm of posterior roll-back of the lateral condyle is seen, leading to the nor- mal axial rotation of the femur relative to the tibia [10].
The present study has determined that on average subjects with a medial UKA experienced a normal AP kinematic pattern, but less pronounced in magnitude than in the normal knee, with the medial condyle re- maining in a similar contact position throughout a deep knee bend and during stance phase of gait. On average, subjects with a medial UKA experienced only 0.8 mm of average motion during stance phase of gait and 11 of 15 subjects experienced either 2.0 mm or less motion, simi- lar to the range for the normal knee. The results for the few lateral UKAs showed more variability, with the later- al condyle either moving in the posterior direction or demonstrating minimal motion. During gait, subjects with a medial UKA experienced more normal axial rota- tion patterns than subjects with a lateral UKA, with 0.9°
of normal axial rotation for medial UKA (compared with 4.0° for the normal knee) from heel-strike to toe-off, while subjects with a lateral UA experienced -6.0° of opposite axial rotation.
The results of this study may suggest that for some subjects progressive laxity of the ACL may occur over time [11]. This finding was very similar to what was re- ported based on fluoroscopic evaluations of PCL-retain- ing TKA in which the ACL is absent [9]. The functional role of the ACL for maintaining satisfactory knee kine- matics has been previously demonstrated both in cadav- eric and clinical studies [12, 13]. The deficient ACL func- tion found in these UKA patients may be related to vari- ous factors such as unrecognized tears at the time of operation, or chronic attenuation occurring after im- plantation secondary to minor traumata, or by abrasion
caused by persistent notch osteophytes. The clinical adverse consequence could be a potentially higher risk for accelerated tibial polyethylene wear as demonstrated during laboratory evaluation [14]. Therefore, one of the reasons for premature failure observed in some studies of UKA may be related, at least in part, to kinematic abnormalities secondary to disturbed ACL function.
In conclusion, this study has shown that during weight-bearing activities, such as when performing a deep knee bend or during walking, certain subjects ex- hibited normal knee motion patterns after implantation of UKA, but the amount of motion was less than the mag- nitudes reported for the normal knee. For some subjects abnormal kinematic patterns were found, possibly relat- ed to progressive dysfunction of the ACL over time. The subjects in this study who exhibited normal kinematic patterns demonstrated PFR and internal tibial rotation with progressive flexion, similar to the normal knee.
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