Internal femoral component rotation adversely influences load transfer in total knee arthroplasty: a cadaveric navigated study using the Verasense device
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Purpose and hypothesis
Correct femoral component rotation at knee arthroplasty influences patellar tracking and may determine function at extremes of movement. Additionally, such malrotation may deleteriously influence flexion/extension gap geometry and soft tissue balancing kinematics. Little is known about the effect of subtle rotational change upon load transfer across the tibiofemoral articulation. Our null hypothesis was that femoral component rotation would not influence load across this joint in predictable manner.
A cadaveric study was performed to examine load transfer using the orthosensor device, respecting laxity patterns in 6° of motion, to examine load across the medial and lateral compartments across a full arc of motion. Mixed-effect modelling allowed for quantification of the effect upon load with internal and external femoral component rotation in relation to a datum in a modern single-radius cruciate-retaining primary knee design.
No significant change in maximal laxity was found between different femoral rotational states. Internal rotation of the femoral component resulted in significant increase in medial compartment load transfer for knee flexion including and beyond 60°. External rotation of the femoral component within the limits studied did not influence tibiofemoral load transfer.
Internal rotation of the femoral component will adversely influence medial compartment load transfer and could lead to premature polyethylene wear on the medial side.
KeywordsFemoral rotation Load Flexion Knee arthroplasty
Total knee arthroplasty (TKA) is designed to alleviate pain and restore function with optimal active range of movement. Contemporary primary knee replacement is met with 10-year survivorship in excess of 95%, up to 20% of the patient cohort remain less than entirely satisfied and early revision for loss of movement and pain is a significant clinical worry [1, 21, 35, 41]. In both registry and single-institution series, up to 20% of the patient group remain dissatisfied with the final outcome with greater than 50% of such cases related to poor movement or lack of stability and unexplained pain . Modern designs of cruciate-retaining knee replacement systems aim to distribute load evenly across the articulating surfaces so as to both reduce polyethylene wear rates and ultimately the TKA revision burden . The kinematic performance of the artificial knee is reliant upon exact placement with respect to the soft tissue envelope of the tibiofemoral and patellofemoral articulations . The common denominator for both is the femoral component.
Femoral component placement may be performed using either a gap balancing or measured technique [26, 34]. No technique is consistently superior or reliable for femoral component placement . Gap balancing is reliant upon correct tibial resection; otherwise, there is a compound error [14, 22]. Measured resection may introduce a malrotation due to difficulty identifying key anatomical landmarks [25, 43]. Femoral component malrotation will deleteriously influence the geometry of the flexion gap and patellar tracking [2, 17]. Load and constraint enjoy a complex and not always inverse relationship, and the flexion gap works with the soft tissue envelope and final laxity pattern to determine load across the flexed knee [8, 14]. Abnormal load may cause pain and stiffness. Our current standard biomechanical assessment is restricted to standing alignment views, and our knowledge of tibiofemoral load transfer in flexion is limited. A greater understanding of the load distribution across the tibiofemoral articulation under defined laxity conditions would allow for the study of the kinematic effect of component rotation and therefore subsequently predict the clinical performance of such a prosthetic joint .
In this study, we performed work to quantify the effect of femoral component rotation upon knee laxity and tibiofemoral contact force. Our primary (null) hypothesis was that femoral component rotation would neither influence load transfer across the tibiofemoral articulation nor maximal laxity for the knee arthroplasty construct at key points of knee flexion.
Materials and methods
Data capture and analysis
Data were captured, as per previous validated work, using a standard computerised navigation system with orthosensor provided range of compatible tibial trials [14, 37]. Knees were manually stressed to mimic intraoperative laxity assessment. A datum was taken from the knee in extension from which maximal displacements of the tibia in relation to the fixed femur were tracked via computer navigation (Stryker eNdtrac Knee Navigation System, Michigan, USA) to an accuracy of ±0.5 mm in 6° of freedom [5, 10, 28]. For each TKA condition, maximal displacements (anteroposterior, varus, valgus, internal and external rotation) were each recorded at five angles of flexion (0°, 30°, 60°, 90° and 110°). To reduce hysteresis, repeated flexion cycles were undertaken between measurements and ensured compartment forces remained constant during passive flexion. After each set of measurements, the output instrumentation was reset at zero. The Verasense device recorded tibiofemoral contact force (lbs/force) and contact points continuously during testing (millimetre accuracy ±2 mm—C. Anderson, OrthoSensor, personal communication 03.08.2015) with additional data capture under maximal stress at each of the five angles of flexion. Three knee conditions were defined with the internal and external rotatory states compared with the neutral or datum. These were the internally rotated femoral component (IRF-TKA) and the externally rotated femoral component (ERF-TKA) (Fig. 3).
Mixed-effect modelling was used to quantify the effect of flexion angle, direction of movement and implantation of TKA upon laxity [30, 32]. Displacements were used as the response variable, with TKA and flexion as covariates. Student’s t test was used to compare differences in tibiofemoral force and contact point measurements. Significance was set at a level of p < 0.05.
CR-TKA laxity pattern
A decrease in laxity was found for all knees after implantation of a single-radius (CR-TKA) cruciate-retaining knee design when compared to the native state. This reduction in maximal laxity only reached significance for rotatory laxity in higher levels of knee flexion beyond 90° (Fig. 4a–c).
IRF-TKA laxity pattern
Maximal anteroposterior was significantly increased for the internally rotated femoral component state beyond 60° when compared to both the neutrally and externally rotated femoral component states. Varus valgus maximal movement and maximal rotatory motion assessment failed to identify any difference for the internally rotated component state when compared to neutral or external rotation (Fig. 4a–c).
Load across the medial compartment was significantly increased at and beyond 60° of flexion with the internal femoral component state (Fig. 5b). This was most marked when stressing the knee with maximal internal rotation beyond 60° of flexion. At 90° flexion, a mean of 83% of the total contact load was borne by the medial compartment in the ERF-TKA compared to 47 and 43% in the CR-TKA and ERF-TKA, respectively (p < 0.05).
ERF-TKA laxity pattern
The laxity pattern for the externally rotated femoral component knee arthroplasty (ERF-TKA) did not significantly alter when compared to the primary neutral position of the femoral component. Whilst the greatest difference between the ERF-TKA and CR-TKA was found at 110° with an increase in rotational laxity, this failed to reach statistical significance. No significant difference at any angle of flexion was found between the neutrally and externally rotated femoral component arthroplasty states for maximal anteroposterior movement or varus valgus stressing (Fig. 4a–c).
Similar load was found across the medial and lateral femoral compartments between 0° and 30° of flexion when comparing the ERF-TKA versus CR-TKA states. Between 60° and 110° load increased across the lateral compartment under rotational stress for the ER-TKA when compared to the CR-TKA. This change did not, however, reach significance (Fig. 5c). It was representative of a proportional increase in lateral compartment loading in the ER-TKA state compared to the CR-TKA in mid to deep flexion. Overall, the ERF-TKA load pattern very closely resembled the pattern of contact force recorded for the neutrally aligned femoral component (CR-TKA) state.
Load transfer with varus/valgus stress testing
There was consistency of load transfer across the medial and lateral compartments for both neutral datum femoral position and external rotation of the femoral component under both valgus (Fig. 5d) and varus maximal stress testing (Fig. 5e). Interesting, whilst this did not reach significance, there was a consistent increase in load transfer on the medial compartment in higher degrees of knee flexion for both varus and valgus load (Fig. 5d, e).
The principal findings of this work were the rejection of the null hypothesis that femoral component rotation would not influence load transfer across the tibiofemoral joint. Internal rotation of the femoral component led to increased load across the medial tibiofemoral compartment most markedly beyond mid-flexion. This redistribution of load was found even with a neutrally aligned knee replacement. It is possible that such increased load across the medial compartment could explain the pain reported by patients in clinical series with malrotated components [3, 17, 29]. The study failed to demonstrate a reciprocal increase in load on the lateral side with external rotation of the femoral component. Further, whilst the relationship between laxity patterns for total load across the tibiofemoral articulation mimicked those previous reported, the experiment failed to demonstrate a significant inverse relationship of such at the extremes of motion. Subtle internal malrotation of the femoral component resulted in a substantial increase in load transfer across the medial compartment. These differences were most evident for both varus and valgus stress testing in flexion. This dynamic biomechanical imbalance in flexion would not be detected in any standard long leg film view performed postoperatively and sheds light on the limitations of static views to determine biomechanical axes after surgery.
There are weaknesses inherent in the use of cadaveric material for such experimental work. None of the knees used in this work exhibited arthritic change or deformity. And whilst this does minimise the confounding effects of soft tissue contracture or bone loss, it may not entirely accurately replicate the operative state. However, the use of non-arthritic knees did allow for better accuracy when determining anatomical landmarks such as the epicondylar axis and ensured that baseline implantation was simple reliable and achieved quickly by the two senior operating surgeons. In contrast to previous work, all muscle groups acting across the knee joint were loaded as per previous methodology . Particular attention was made to ensure free full tibiofemoral and patellar movement in the native loaded state and after primary implantation, cognisant of previous work, thereby confirming the importance of load across the joint to achievement of inherent stability [16, 22, 46]. The loads used in this study were subphysiological consistent with previous work so as to reduce risk of muscle tearing. Time zero assessment takes no account of the patient size, demographics or activity level. However, the use of cadaveric knees does allow for repeated-measures statistical analyses and reduces the confounding influence of pathological or functional differences between patients, thereby potentially enhancing the power of this work. Furthermore, prior to experimentation, all limbs had undergone radiographic assessment to exclude peri-articular deformity, degenerative change and confirmed neutral frontal plane alignment, which was confirmed as neutral for all limbs.
Excess internal femoral rotation in the absence of tibial malrotation is reported as an indication for revision arthroplasty [13, 23]. Several series confirm that revision for femoral component malrotation in isolation will achieve improved function with particular emphasis on functional measures which rely upon a stable and balanced mid-flexion tibiofemoral articulation [13, 23, 42]. Unlike the work of Thompson et al., we did allow for the tibia to sit on the tibial surface where there was maximal conformity, by cycling the knee after fixing the femoral component and allowing the tibia to sit accordingly, congruity was optimised from a load perspective [7, 33, 40]. Internal femoral component rotation with overstuffing of the medial compartment in flexion may stretch the MCL leading to pain, stiffness, failure to facilitate safe flexion and impaired varus/valgus laxity [11, 22, 33, 40]. Increased compartmental load on the medial side may lead to premature failure and suboptimal performance due to abnormal non-physiological load being shared across the articulation. Jeffcote et al.  correlated tibiofemoral forces with collateral ligament strain when increasing the flexion gap. Medial tibial pain has been reported in this patient cohort, and the finding of increased medial compartmental load with internal femoral component rotation could offer an explanation for such. We believe that this is the first work to identify abnormal preferential asymmetrical loading of the medial compartment without evidence of a reciprocal effect on laxity. Much previous work has argued that achievement of a quadrilateral flexion gap and the use of ligament tension devices may aid with load distribution but without determining load per se. However, this work does raise concerns about how reliable peroperative manual stress is at determining equivalent loading across the medial and lateral compartments . Previous CT work has found rotational alignment errors in vivo to range from 13° of internal rotation to as much as 16° of external rotation have been documented . Correct coronal plane alignment is associated with good clinical outcomes, but little is known of the correct mechanical axis in flexion . Our work has focused on a much narrower range of rotation and despite such has still identified significant variation in load transfer. Greater errors in 3D placement can underscore the early clinical failures with patients reporting suboptimal functional performance from mediolateral pathological laxity and inevitable pain from loosening and abnormal loading of the subchondral bone .
Our work failed to find differences in load on the lateral side with external rotation of the femoral component. It is known that the posterolateral corner, additionally, is ineffective in flexion . This apparent capacity to dissipate load in flexion could explain the minimal change in load on the lateral side with excess femoral external rotation. Recent work on the kinematics of femoral component rotation did examine the impact of femoral and tibial component rotation upon load in the surrounding muscle groups . These workers found that femoral component rotation principally influenced load in the quadriceps apparatus and collateral ligaments, thereby determining varus valgus movement. However, no work was done to examine load across the joint. Our work has added to this knowledge by relating in isolation femoral component rotation, albeit with more subtle rotational margins .
Compliance with ethical standards
Conflict of interest
There are no conflicts of interest as per previous cadaveric work.
Funding was provided by Newcastle Charitable Trustees and Stryker Research Fund Europe.
As per HTA licence copied to Newcastle Hospital / Newcastle University for the nstc http://www.nstcsurg.org.uk, ethical approval is institutional.
No informed consent was required as no patient material was used and no patient was involved in this work.
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