Prosthesis alignment affects axial rotation motion after total knee replacement: a prospective in vivo study combining computed tomography and fluoroscopic evaluations
© Harman et al.; licensee BioMed Central Ltd. 2012
Received: 18 January 2012
Accepted: 20 September 2012
Published: 23 October 2012
Clinical consequences of alignment errors in total knee replacement (TKR) have led to the rigorous evaluation of surgical alignment techniques. Rotational alignment in the transverse plane has proven particularly problematic, with errors due to component malalignment relative to bone anatomic landmarks and an overall mismatch between the femoral and tibial components’ relative positions. Ranges of nominal rotational alignment are not well defined, especially for the tibial component and for relative rotational mismatch, and some studies advocate the use of mobile-bearing TKR to accommodate the resulting small rotation errors. However, the relationships between prosthesis rotational alignment and mobile-bearing polyethylene insert motion are poorly understood. This prospective, in vivo study evaluates whether component malalignment and mismatch affect axial rotation motions during passive knee flexion after TKR.
Eighty patients were implanted with mobile-bearing TKR. Rotational alignment of the femoral and tibial components was measured from postoperative CT scans. All TKR were categorized into nominal or outlier groups based on defined norms for surgical rotational alignment relative to bone anatomic landmarks and relative rotational mismatch between the femoral and tibial components. Axial rotation motion of the femoral, tibial and polyethylene bearing components was measured from fluoroscopic images acquired during passive knee flexion.
Axial rotation motion was generally accomplished in two phases, dominated by polyethylene bearing rotation on the tibial component in early to mid-flexion and then femoral component rotation on the polyethylene articular surface in later flexion. Opposite rotations of the femur-bearing and bearing-baseplate articulations were evident at flexion greater than 80°. Knees with outlier alignment had lower magnitudes of axial rotation and distinct transitions from external to internal rotation during mid-flexion. Knees with femoral-tibial rotational mismatch had significantly lower total axial rotation compared to knees with nominal alignment.
Maintaining relative rotational mismatch within ±5° during TKR provided for controlled knee axial rotation during flexion. TKR with rotational alignment outside of defined surgical norms, with either positive or negative mismatch, experienced measurable kinematic differences and presented different patterns of axial rotation motions during passive knee flexion compared to TKR with nominal mismatch. These findings support previous studies linking prosthesis rotational alignment with inferior clinical and functional outcomes.
Clinical Trials NCT01022099
KeywordsTotal knee replacement Mobile-bearing prosthesis Implant alignment Surgical alignment Knee kinematics Axial rotation Knee biomechanics Knee arthroplasty
Attaining proper prosthesis alignment during total knee replacement (TKR) is essential for stable TKR function and successful clinical outcomes[1–5]. The associated technical challenges have led to the rigorous evaluation of surgical alignment techniques for identifying anatomic landmarks and defining the joint axes during TKR surgery[6–13]. However, deviation from optimal alignment persists in some cases, especially in the transverse plane (rotational alignment)[14–17]. Furthermore, optimal rotational alignment of the femoral and tibial components relative to fixed anatomic landmarks can still produce complications due to an overall mismatch in rotational alignment of the femoral component relative to the tibial component[14, 16, 18–20].
TKR designs with mobile polyethylene bearings are advocated for their professed ability to self-align and accommodate small rotational alignment errors[19, 21–24]. Such errors can include rotational malalignment with respect to bone anatomic landmarks, as well as mismatch between the relative positions of the femoral and tibial components. However, for many mobile-bearing TKR, understanding the relationships between prosthesis rotational alignment, knee axial rotation motion, and polyethylene bearing motion is difficult[24–28]. Consequently, it remains largely unknown whether knee axial rotation is accomplished through femoral component motion on the bearing articular surface or through bearing motion on the tibial baseplate.
This study addresses the following specific research question. Does component malalignment affect knee axial rotation motion and bearing motion in mobile-bearing TKR? The objective was to assess TKR rotational alignment, knee axial rotation motion and polyethylene bearing motion that occur in vivo during passive flexion in subjects with mobile-bearing TKR. It was hypothesized that TKR with rotational alignment within defined surgical norms would present different knee axial rotation motion and bearing motion compared to TKR with rotational alignment outside surgical norms.
Patient demographics and preoperative clinical data for entire randomized subject population and cohort included in the kinematic analysis (medians and range for continuous data, absolute and relative frequencies for categorical data)
69 (47 – 87)
69 (47 – 84)
Sex (% female)
84 (60 – 146)
85 (62 – 146)
Body Mass Index (kg/m 2 )
29.7 (22.0 – 47.7)
30.0 (22.0 – 47.7)
Based on these CT measurements, all TKR were categorized according to nominal tolerances for surgical rotational alignment using criteria based on bone anatomy (Anatomic Landmarks group) or relative rotational alignment between the femoral and tibial components (Rotational Mismatch group). The tolerances were established using both surgical norms and a clinical perspective, since acceptable tolerance for tibial rotational alignment and rotational mismatch between the components are not well-defined[10, 15, 16, 18, 20, 31–34]. In the Anatomic Landmarks group, TKR were categorized as having “nominal” rotational alignment with respect to anatomic landmarks if the alignment was within ±3° for the femoral components and ±10° for the tibial components. TKR exceeding these limits were categorized as “outliers”. In the Rotational Mismatch group, TKR were categorized as having “nominal” rotational mismatch if the relative femoral-tibial rotational mismatch was within ±5°. TKR exceeding these limits were categorized as “outliers”.
These measurements from fluoroscopic images determined the relative axial rotation motion for all three prosthesis components. Total knee axial rotation was defined as relative internal-external motion between the femoral component and metal tibial baseplate in the transverse plane. Articular axial rotation was defined as relative motion occurring at the articular surface between the femoral component and the polyethylene bearing. Bearing axial rotation was defined as relative motion occurring at the distal backside surface between the polyethylene bearing and the tibial baseplate. Positive axial rotation corresponded to femoral internal rotation (tibial external rotation) and negative axial rotation corresponded to femoral external rotation (tibial internal rotation).
The primary endpoints of this study were knee axial rotation motion during passive knee flexion, including the magnitude of total axial rotation, articular axial rotation, and bearing axial rotation. The normal and outlier categories within each of the two groups defined by the CT measurements (Anatomic Landmarks; Rotational Mismatch) were compared for differences in clinical factors (patients’ age, weight, body mass index, Knee Society Scores), limb alignment (mechanical axis), and TKR rotational alignment. The relationships between axial rotation motion and flexion angle for the normal and outlier categories within each group also were compared. Sample size was determined in the initial clinical study and was based on the ability to detect differences of 5 degrees in the components’ rotational alignment. Statistical analysis software (SigmaStat version 2.03, SPSS Inc., Chicago, Illinois USA) was used for data processing to execute analysis of variance with appropriate post-hoc multiple comparisons, including non-parametric methods when applicable.
Rotational alignment of components (median, range) measured from postoperative CT images for alignment groups defined with respect to anatomic landmarks (surgical transepicondylar axis, medial third of the tibial tuberosity) and rotational mismatch between femoral and tibial components
n (% total)
Rotational alignment (°)
0.6 (−2.8 – 2.7)
0.0 (−6.5 – 9.9)
0.4 (−9.5 – 10.6)
1.2 (−3.4 – 3.8)
12.0 (−14.9 – 26.0)
−0.5 (−13.3 – 14.4)
p = 0.554
p = 0.002
p = 0.846
0.6 (−3.4 – 3.8)
0.0 (−13.3 – 26.0)
−0.2 (−5.0 – 4.6)
0.9 (−3.1 – 3.5)
0.0 (−14.9 – 20.9)
6.2 (−13.0 – 14.4)
p = 0.659
p = 0.084
p = 0.146
Based on CT measurements, approximately one-third of the TKR had rotational alignment outside of defined tolerances for component alignment in both the Anatomic Landmarks and Rotational Mismatch groups (Table2). Tibial components showed a greater variance in rotational alignment compared to femoral components and contributed to a greater proportion of TKR identified as outliers. There were 16 (24%) TKR with isolated malrotation of the tibial component, 4 (6%) with isolated malrotation of the femoral component, and 1 (1%) with malalignment of both the tibial and femoral components. Ten TKR were identified as being outliers in both the Anatomic Landmark group and the Rotational Mismatch group. Outlier TKR in the Anatomic Landmarks group had 12.0° more tibial internal rotation alignment compared to nominal TKR. Outlier TKR in the Rotational Mismatch group included nine TKR with negative mismatch (femoral external rotation relative to the tibial component) and 14 TKR with positive mismatch (femoral internal rotation relative to the tibial component), resulting in 6.4° more femoral-tibial mismatch biased toward tibial external rotation (femoral internal rotation) compared to the nominal TKR.
In the Anatomic Landmarks group, different patterns of axial rotation motion occurred over the flexion range for the nominal and outlier groups (Figure4). Among TKR categorized as nominal (46 knees), there was a gradual increase in total axial rotation with flexion, consistent with increasing femoral external rotation. In general, axial rotation from 0° to approximately 80° occurred primarily due to external rotation of the polyethylene bearing on the tibial baseplate. However, for TKR categorized as outliers (21 knees), there was a distinct transition from external to internal rotation from 20° to 50° of flexion and a second distinct transition into external rotation from 50° to 80° of flexion (Figure4). Beyond 80°, both nominal and outlier TKR showed combined polyethylene bearing axial rotation and external rotation of the femoral component on the polyethylene articular surface, with the latter dominating the motion pattern. Significant differences in total axial rotation for the nominal and outlier groups were not detected (p > 0.05) for the increments of flexion. However, statistical power was limited (β<0.8) for these comparisons and the lack of observed statistical differences should be interpreted with caution.
This study combined CT and fluoroscopic imaging during flexion to address the consequences of component malalignment on the in vivo motion of mobile-bearing TKR. Prosthesis rotational alignment outside of defined surgical tolerances significantly affected the knee axial rotation motion and bearing motion during passive flexion. In these subjects with mobile-bearing TKR, axial rotation was generally accomplished in two phases over the passive flexion range, dominated by polyethylene bearing rotation on the tibial component in early to mid-flexion and then femoral component rotation on the polyethylene articular surface in later flexion (Figures4 and5). However, TKR with rotational alignment outside of surgical norms presented different patterns of axial rotation motions, including distinct transitions in bearing rotational motion during mid-flexion, lower magnitudes of total external rotation, and opposite rotations of the femur-bearing and bearing-baseplate articulations (Figures4 and5).
Intraoperative assessment of passive range of motion and CT imaging of the components’ alignment proved useful for measuring in vivo tibial-femoral kinematics and objectively categorizing the nominal and outlier alignment groups after mobile-bearing TKR. A limitation with this study is that the measured kinematics are reflective of passive knee flexion, without muscle contraction or weight-bearing by the patient. This motion was evaluated in an effort to capture the effect of surgical technique and rotational alignment, without adding variability due to patient habitus, dynamic activity and possible pain. Furthermore, using slow and controlled movement of the knee eliminated measurement difficulties that can occur with motion blur in the image frames.
This mobile-bearing prosthesis design accomplished axial rotation in two phases, including external rotation of the polyethylene bearing on the tibial baseplate in early to mid-flexion and external rotation of the femoral component on the polyethylene articular surface at flexion >80°. Abrupt transitions in bearing rotation were observed and were most pronounced for outlier TKR in both the Anatomic Landmarks and Rotational Mismatch groups (Figures4 and5). Bearing motion did not always follow femoral motion for this activity and nonconforming mobile bearing design, consistent with other in vivo studies of bearing motion[25, 36, 39]. These data characterize a decoupling of the femoral and tibial components’ rotations, similar to the patterns observed in a dynamic musculoskeletal model of mobile-bearing TKR during simulated squatting.
Maintaining rotational mismatch within ±5° during TKR provided for controlled femoral external rotation motion occurring with passive flexion. In contrast, bearing motions in the mid-flexion range of motion were distinctly different among outlier TKR in both the Anatomic Landmarks and Rotational Mismatch groups compared to nominal TKR (Figures4 and5). This may have consequences for dynamic activities that demand stability during mid-flexion when joint loads due to muscle contraction are high. Furthermore, external rotation was essentially arrested beyond 80° in Anatomic Landmarks outliers, which can interfere with patella function[14, 18, 34, 40]. In the Rotational Mismatch group, outlier TKR alignment was biased more than 6° toward femoral internal rotation relative to the tibial component (Table2), resulting in significantly less total axial rotation (decreased external rotation motion) compared to nominal TKR (Figure5). A similar reduction in axial rotation motion with femoral-tibial component mismatch biased toward femoral internal rotational alignment has been observed during in-vitro testing of cadaver limbs loaded to simulate rising from a chair.
The nominal tolerances for component alignment and rotational mismatch in the transverse plane remain under debate[8, 11–16, 18–22]. In the current study, TKR were categorized as nominal if component alignment relative to anatomic landmarks was within ±3° for femoral components and within ±10° for tibial components, and if relative femoral-tibial mismatch was within ±5°. These ranges were established based on reported surgical precision for achieving targeted component alignment[10, 15, 16] and the magnitude of deviation from optimal alignment that has been associated with clinical and biomechanical complications[18, 20, 31–34].
Femoral component alignment deviating from ±3° was considered as outlier alignment since it does not represent precise surgical technique and has the potential to contribute to poor outcomes. Several studies report that ±3° precision for femoral component rotation is readily achieved in more than 85% of TKR[10, 15, 16]. Femoral component rotation exceeding approximately ±5° has been associated with clinical complications, including pain and patellar failure. It is recognized that precise tibial component axial rotation relative to anatomic landmarks is difficult to achieve. Tibial component alignment deviating from ±10° was considered as outlier alignment since it exceeds surgical norms and has the potential to contribute to poor outcomes. Reported alignment precision for tibial components exceeds ±3° in approximately 50% of TKR and exceeds ±10° in approximately 30% of TKR. Absolute mean deviations of 3° to 8° of tibial component axial rotation alignment have been reported,[10, 15] with pain and patellar dislocation and failure associated with deviations exceeding approximately 10°.
While several studies report combined rotation and rotational mismatch between the femoral and tibial components after TKR[14, 18, 32, 41], few report clinical consequences associated with these parameters. Adverse consequences associated with approximately 10° of combined rotation or rotational mismatch include no improvement in Knee Society function scores,, knee pain, and patellar dislocation or failure. In the current study, 7 of the 23 TKR categorized as outliers in the Rotational Mismatch group had rotational mismatch exceeding ±10° and those patients previously were reported to exhibit no functional improvement. Expanding the current analysis to include TKR with rotational mismatch exceeding ±5° shows that even smaller magnitudes of mismatch can have significant biomechanical consequences (Figure6B).
Obtaining alignment within the above defined nominal ranges provided for controlled knee axial rotation (Figure4 and5). However, these tolerances were exceeded in 31% and 34% of the TKR when evaluated relative to anatomic landmarks and rotational mismatch, respectively (Table2). Rotational alignment of the tibial components proved especially variable and contributed to these relatively high percentages of outliers, similar to our previous report, as surgical techniques referencing the tibial tubercle have proven inconsistent[15–17]. The observed variations in surgical rotational alignment provide some explanation for the highly variable bearing motions that have been observed in vivo for various mobile-bearing TKR designs[24–27, 36–39].
Clinical consequences for TKR patients with isolated and combined internal rotation alignment of the femoral and tibial components include anterior knee pain and patellar complications[14, 18, 21, 32]. In a series of failed TKR with patellofemoral complications, 3°–8° of internal rotation malalignment was correlated with patellar subluxation and 7°–17° of internal rotation malalignment was correlated with early patellar dislocation or late prosthesis failure. Barrack, et al. found 6.8° more internal rotational alignment of the tibial component in patients with anterior knee pain, both with and without patellar resurfacing, compared to control patients without pain. Compared to nominal TKR in the current study, outliers in the Anatomic Landmarks group showed a bias of 12° of internal rotation of the tibial component, and outliers in the Rotation Mismatch group showed a bias of 6.4° more femoral internal rotation (tibial external rotation) malalignment (Table2). Careful monitoring of the mid-term clinical outcomes for all patients in the current study cohort is ongoing, with preliminary data showing a trend toward worse Knee Society function scores in patients with more than 10° of relative rotational alignment between the femoral and tibial components at a median follow-up time of 20 months. Therefore, contrary to some studies suggesting that mobile-bearing TKR designs compensate for errors in rotational alignment[17, 19, 21–24], patients with mobile-bearing TKR can experience measurable kinematic differences and worse functional outcomes when rotational alignment is outside of defined surgical norms.
Axial rotation motion was generally accomplished in two phases, dominated by polyethylene bearing rotation on the tibial component in early to mid-flexion and then femoral component rotation on the polyethylene articular surface in later flexion. Maintaining relative rotational mismatch within ±5° during TKR provided for controlled knee axial rotation during flexion. TKR with rotational alignment outside of defined surgical norms experienced measurable kinematic differences and presented different patterns of axial rotation motions during passive knee flexion. These findings support previous studies[14, 18, 21, 32, 41] linking prosthesis rotational alignment with inferior clinical and functional outcomes.
The authors acknowledge Lucia Talavera and Emily Downs for their assistance in image analysis. An institutional research grant from Stryker Orthopaedics was provided to the Orthopaedic Department at the University Hospital Carl Gustav Carus Dresden to fund the costs of mandatory patient insurance policies associated with exposure to additional radiation. An institutional research grant from Stryker Orthopaedics was provided to The Institute for Mobility & Longevity to fund the cost of image analysis. Partial funding to support manuscript writing was received from a Marie Curie International Incoming Fellowship within the 7th European Community Framework Program. None of the funding bodies had a role in the collection, analysis or interpretation of the data, in manuscript writing, or in the decision to submit the manuscript for publication.
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