Aims. This study aimed to analyze kinematics and kinetics of the tibiofemoral joint in healthy subjects with valgus, neutral, and varus limb alignment throughout multiple gait activities using dynamic videofluoroscopy. Methods. Five subjects with valgus, 12 with neutral, and ten with varus limb alignment were assessed during multiple complete cycles of level walking, downhill walking, and stair descent using a combination of dynamic videofluoroscopy, ground reaction force plates, and optical motion capture. Following 2D/3D registration, tibiofemoral kinematics and kinetics were compared between the three limb alignment groups. Results. No significant differences for the rotational or translational patterns between the different limb alignment groups were found for level walking, downhill walking, or stair descent. Neutral and varus aligned subjects showed a mean centre of rotation located on the medial condyle for the loaded stance phase of all three gait activities. Valgus alignment, however, resulted in a centrally located centre of rotation for level and downhill walking, but a more medial centre of rotation during stair descent. Knee adduction/abduction moments were significantly influenced by limb alignment, with an increasing knee adduction moment from valgus through neutral to varus. Conclusion. Limb alignment was not reflected in the condylar kinematics, but did significantly affect the knee adduction moment. Variations in frontal plane limb alignment seem not to be a main modulator of condylar kinematics. The presented data provide insights into the influence of anatomical parameters on tibiofemoral kinematics and kinetics towards enhancing clinical decision-making and surgical restoration of natural knee joint motion and loading. Cite this article: Bone Joint Res 2024;13(9):485–496

Aims

While mechanical alignment (MA) is the traditional technique in total knee arthroplasty (TKA), its potential for altering constitutional alignment remains poorly understood. This study aimed to quantify unintentional changes to constitutional coronal alignment and joint line obliquity (JLO) resulting from MA.

Background. Advanced technologies, like robotics, provide enhanced precision for implanting total knee arthroplasty (TKA) components; however, optimal component position and limb alignment remain unknown. This study purpose was to identify the ideal target sagittal component position and coronal limb alignment that produce optimal clinical outcomes. Methods. A retrospective review of 1,091 consecutive TKAs was performed. All TKAs were PCL retaining or sacrificing with anterior lipped (49.4%) or conforming bearings (50.6%) performed with modern perioperative protocols. Posterior tibial slope, femoral flexion, and tibiofemoral limb alignment were measured with a standardized protocols. Patients were grouped by the ‘how often does your knee feel normal?’ outcome score at latest follow-up. Machine learning algorithms were used to identify optimal alignment zones which predicted improved outcomes scores. Results. Mean age and BMI were 66 years and 34 kg/m. 2. with 67% female. Demographics and relevant covariates did not affect outcomes (p≥0.145) except for BMI (p=0.077) but the difference was not clinically significant. For sagittal alignment, approximating native tibial slope within 0 to +2° with some amount of femoral flexion within 0 to +3° (possibly up to +9°) was predictive of knees always feeling normal. For knees in preoperative varus or neutral, knees were more likely to always feel normal when postoperative tibiofemoral alignment was in varus (>−1°). Knees aligned in valgus preoperatively were more likely to always feel normal in valgus (<−7°) or varus (>−4°) postoperatively. Conclusion. Superior patient-reported outcomes correlated with approximating native tibial slope and incorporating some femoral flexion while maintaining similar preoperative coronal limb alignment. Excessive deviation from native tibial slope, excessive femoral flexion or any femoral component extension, or coronal alignment overcorrection beyond the preoperative limb alignment correlated with worse outcomes

Excessive under correction of varus deformity may lead to early failure and overcorrection may cause progressive degeneration of the lateral compartment following medial unicompartmental knee arthroplasty (UKA). However, what influences the postoperative limb alignment in UKA is still not clear. This study aimed to evaluate postoperative limb alignment in minimally-invasive Oxford medial UKAs and the influence of factors such as preoperative limb alignment, insert thickness, age, BMI, gender and surgeon's experience on postoperative limb alignment. Clinical and radiographic data of 122 consecutive minimally-invasive Oxford phase 3 medial unicompartmental knee arthroplasties (UKAs) performed in 109 patients by a single surgeon was analysed. Ninety-four limbs had a preoperative hip-knee-ankle (HKA) angle between 170°-180° and 28 limbs (23%) had a preoperative hip-knee-ankle (HKA) angle <170°. The mean preoperative HKA angle of 172.6±3.1° changed to 177.1±2.8° postoperatively. For a surgical goal of achieving 3° varus limb alignment (HKA angle=177°) postoperatively, 25% of limbs had an HKA angle >3° of 177° and 11% of limbs were left overcorrected (>180°). Preoperative HKA angle had a strong correlation (r=0.53) with postoperative HKA angle whereas insert thickness, age, BMI, gender and surgeon's experience had no influence on the postoperative limb alignment. Minimally invasive Oxford phase 3 UKA can restore the limb alignment within acceptable limits in majority of cases. Preoperative limb alignment may be the only factor which influences postoperative alignment in minimally-invasive Oxford medial UKAs. Although the degree of correction achieved postoperatively from the preoperative deformity was greater in limbs with more severe preoperative varus deformity, these knees tend to remain in more varus or under corrected postoperatively. Overcorrection was more in knees with lesser preoperative deformity. Hence enough bone may need to be resected from the tibia in knees with lesser preoperative deformity to avoid overcorrection whereas limbs with large preoperative varus deformities may remain under corrected

Component malalignment has long been implicated in poor implant survival in Total Knee Arthroplasty (TKA). Malalignment can occur in orientation of bony cuts, and in component cementation/implantation. Several systems exist to aid bony cut alignment (navigation, shape matching), but final implantation technique is common to all TKA. Correction of errors in bony cut alignment at cementation/implantation by surgeons has been described. Changes in alignment at this stage are likely to result in asymmetrical cement penetration, which is implicated in early failure. This study reviewed a consecutive series of 150 primary cemented TKAs using an imageless navigation system (aiming for neutral overall limb alignment). Deviation at implantation was calculated by comparing limb alignment recorded using the trial components with limb alignment recorded with the final implanted components, prior to closure. 136 patients (91%) had a final overall limb alignment within 2° of neutral. Three patients (2%) had a final overall limb alignment greater than 3° from neutral. Deviation occurring at implantation is shown in Figure 1 with deviations distributed around zero (mode 0, median 0.3, range −2 to +4,)

Introduction. Component and limb alignment (especially varus >3°) have been associated with soft-tissue imbalance, increased polyethylene wear, and tibial tray subsidence. However, not all clinical outcome studies have found significant correlation between tibial varus and revision surgery. While the link between limb alignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. In this study, we analyzed the effect of limb alignment and tibial tray alignment on the risk for bone damage and subsequent risk for tray loosening. Methods. A finite element model of knee arthroplasty previously validated with in vitro cadaver testing was used. Models of four subjects were constructed with tibial resections simulating a 0°, 3°, 5°, and 7° varus alignment with respect to the mechanical axis of the tibia and the tray implanted at the corresponding angles. Tibial tray orientation was simulated without change in limb alignment (i.e. maintaining the mechanical axis of the knee at 0°) and with limb alignment ranging from 3° valgus to 7° varus (Fig 1). A static load equivalent to three times the bodyweight of the subject was applied in line with the mechanical knee axis. Relative motion between the tibial tray and tibial bone was calculated. Elements with an equivalent von Mises strain >0.4% were selected and assigned an elastic modulus of 5 MPa to reflect damaged bone. Simulation was repeated and after-damage micromotion recorded. Results. At neutral limb alignment, average tray micromotion was <10 μm and did not increase significantly with increasing tray varus (Fig 2). The after-damage micromotion also did not increase significantly. However, limb alignment had a more substantial effect on before- and after-damage micromotion (Fig 3). The magnitude of micromotion increased with increasing varus limb alignment. Discussion. We did not find significant increase in micromotion with increased tray varus (of up to 7°) as long as neutral limb mechanical axis was maintained by compensating for tibial varus with femoral valgus. The volume of bone at risk also did not increase significantly with increasing tray varus. Removing the damaged bone did little to affect after-damage micromotion. This suggests that the “damaged” bone was not an important factor and likely did not contribute to the stability of the tray under the loading conditions analyzed in this report. Changes in limb alignment significantly offset the net axial load vector resulting in damage in a greater volume of elements due to overloading. This is due to the shift in Mechanical axis and load vector with subsequent increase in moment applied to the model. The micromotion was also substantially increased after the damage indicating that the damaged bone was providing structural support to the tray. This emphasizes the effects of increasing the static coronal loading in this model. Consequently, it identifies the benefit of neutral limb alignment in this loading scenario. This model is an extremely valuable tool in studying the effect of surgical alignment, loading, and activity on damage to proximal bone

Aims. The aims of this retrospective study were to determine the incidence of extra-articular deformities (EADs), and determine their effect on postoperative alignment in knees undergoing mobile-bearing, medial unicompartmental knee arthroplasty (UKA). Patients and Methods. Limb mechanical alignment (hip-knee-ankle angle), coronal bowing of the femoral shaft and proximal tibia vara or medial proximal tibial angle (MPTA) were measured on standing, full-length hip-to-ankle radiographs of 162 patients who underwent 200 mobile-bearing, medial UKAs. Results. Incidence of EAD was 7.5% for coronal femoral bowing of >5°, 67% for proximal tibia vara of >3° (MPTA<87°) and 24.5% for proximal tibia vara of >6° (MPTA<84°). Mean postoperative HKA angle achieved in knees with femoral bowing ≤5° was significantly greater when compared to knees with femoral bowing >5° (p=0.04); in knees with proximal tibia vara ≤3° was significantly greater when compared to knees with proximal tibia vara >3° (p=0.0001) and when compared to knees with proximal tibia vara >6° (p=0.0001). Conclusion. Extra-articular deformities are frequently seen in patients undergoing mobile-bearing medial UKAs, especially in knees with varus deformity>10°. Presence of an EAD significantly affects postoperative mechanical limb alignment achieved when compared to limbs without EAD and may increase the risk of limbs being placed in varus>3° postoperatively. Clinical Relevance. Since the presence of an EAD, especially in knees with varus deformity>10°, may increase the risk of limbs being placed in varus>3° postoperatively and may affect long-term clinical and implant survival outcomes, UKR in such knees should be performed with caution

Background. Post-operative (postop) lower limb alignment in unicompartmental knee arthroplasty (UKA) has been reported to be an important factor for postop outcomes. Slight under-correction of limb alignment has been recommended to yield a better clinical outcomes than neutral alignment. It is useful if the postop limb alignment can be predicted during surgery, however, little is known about the surgical factors affecting the postop limb alignment in UKA. The purpose of this study was to examine the influence of the medial tibial joint line elevation on postop limb alignment in UKA. Methods. Seventy-four consecutive medial UKAs were enrolled in this study. All the patients received a conventional fixed bearing UKA. Pre-operative (preop) and postop limb alignment was examined using long leg radiograph and lower limb alignment changes were calculated. Femoral and tibial osteotomy thickness were measured during surgery. Medial tibial joint line change was defined as polyethylene thickness minus tibial osteotomy thickness and sawblade thickness (1.27mm). Positive values indicated a tibial joint line elevation. Medial femoral joint line change was defined as femoral distal component thickness (6.5mm) minus femoral distal osteotomy thickness and sawblade thickness. Positive values indicated a femoral joint line reduction. Medial joint distraction width was also calculated by tibial joint line elevation plus femoral joint line reduction. The correlation of lower limb alignment change with polyethylene insert thickness, the medial tibial joint line elevation, femoral joint line reduction, or joint distraction width were analyzed. Results. The mean preop hip-knee-ankle (HKA) angle was 7.1 ± 3.3° in varus and postop was 2.1 ± 3.0° in varus. The mean lower limb alignment change was 5.0 ± 2.6°. The mean polyethylene insert thickness was 8.5 ± 0.8mm, the tibial joint line elevation was 4.4 ± 1.3mm and the medial femoral joint line reduction was 0.0 ± 1.1mm, the joint distraction width was 4.5 ± 1.5mm. The polyethylene insert thickness, the medial tibial joint line elevation, and the joint distraction width were positively correlated with the lower limb alignment change (R=0.27; P<0.05, R=0.47; P<0.001, R=0.53; P<0.001, respectively) (Figure 1a,b,d). There was no correlation between the medial femoral joint line reduction and the lower limb alignment change (Figure 1c). Discussion. The postop limb alignment in total knee arthroplasty (TKA) is determined by the osteotomy angle of the femur and tibia. On the other hand, it has been reported that the postop alignment in UKA is not influenced by the osteotomy angle but by the insert thickness. Our results indicated that the medial tibial joint line elevation and the joint distraction width were more useful to predict lower limb alignment change than the insert thickness itself. Measuring the medial tibial osteotomy thickness during surgery will help surgeon to predict postop lower limb alignment in UKA. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

In Total Knee Arthroplasty (TKA) restoring the mechanical alignment of the knee joint is essential. This can be improved by considering the individual variability in the angle between the mechanical and anatomical axes of the femur (FMA angle). However with the traditional instrumentation and the use of the most common fixed distal femoral resection angle of 6° we assume little or no variation in the FMA angles in different patients. In a previous study we showed that the FMA angle had a wide distribution and that there was a good correlation between the FMA angle and the pre-operative lower limb alignment in the coronal plane. Our hypothesis was that improved post operative limb alignment would be achieved with traditional instrumentation by individual measurement of the FMA angles pre-operatively and adjusting the distal femoral resections accordingly. In the study we compared the post-operative coronal limb alignment for a cohort of patients with a variable distal femoral resection angle to the previous cohort of fixed distal femoral resection angle. The study consisted of 103 patients undergoing 103 consecutive primary TKAs between October 2008 and March 2009. All patients had pre- and post-operative Hip-Knee-Ankle digital radiographs and had TKAs performed using a variable distal femoral cut angle. The FMA angle and the mechanical femoro-tibial (MFT) angles were measured in all cases. Inter-observer variation was measured by second observer readings. We compared our results with the group of 158 consecutive patients undergoing 174 primary TKAs operated between January and October 2007 using fixed distal femoral resection angle. Patient demographics of the two cohorts (age, gender, BMI) were similar. The pre-operative coronal deformity for the variable cohort was less than the fixed, mean 3.7° varus (SD 5.8°) compared to 4.7° varus (SD 7.9°). The FMA angles for the variable cohort ranged from 4° to 8°, (the fixed cohort from 2° to 9°). The variable valgus resection angles cohort showed a correlation between FMA and pre-operative MFT angles as had previously been shown in the fixed cohort (r = −0.499 and r = −0.346 respectively). Post op alignment showed that accuracy within ±5° increased from 86% (fixed resection angle group) to 96% (variable resection group). When using the more commonly quoted accuracy of within ±3°, this changed from 67% (fixed resection angle group) to 85% (variable resection group). These improvements were statistically significant (chi-squared 0.025 and 0.002, respectively). To further evaluate the effect of using variable angles we analysed the improvement of each of the different groups of deformity identified in the previous study (> 8° varus, 8° varus to 1° valgus, > 2° valgus). The range was reduced in both the extreme varus and valgus groups with the variable angles. The most significant improvement was found in the valgus group with the median reducing from 3° to 2° and range from 14° to 8°. It seems logical to use a variable distal femoral resection angle based on the patient’s individual anatomy. By doing so, our results show significant improvement of postoperative limb alignment compared to traditional method of using fixed distal femoral resection angle. In units where preoperative long leg film radiographs are available, measuring the FMA angle and setting the distal femoral resection angle guide accordingly improves the postoperative limb alignment. However, where long leg radiographs are not available, changing the distal femoral resection angle according to the pre-operative varus-valgus deformity is likely to improve the post operative limb alignment. (e.g. 4°–5° distal femoral resection angle for preoperative valgus, 6° for preoperative mild/moderate varus, and 7°–8° for preoperative severe varus).Computer navigation, however, enables us not only to use customised distal femoral cut for each patients, but it also provides many other useful information such as dynamical limb alignment through motion, component rotation, soft tissue balancing

Gap planning in total knee arthroplasty (TKA) navigation is critically concerned. Osteophyte is one of the contributing factors for gap balancing in TKA. The osteophyte is normally removed before gap planning step. However, the posterior condylar osteophyte of femur is sometimes removed during the flexion gap preparation or may not be removed at all depends on individual case. This study attempts to investigate on how posterior condylar osteophyte affects on gap balancing and limb alignment during operation. The study was conducted on 35 varus osteoarthritis knees with posterior condylar osteophyte and undergone on TKA navigation. All knees were measured by CT scan for the size of posterior condylar osteophyte according to its width. Extension gap, flexion gap width, and limb alignment were measured by using the tension device with distraction force of 98 N on both medial and lateral sides under computer assisted surgery. The measuring of extension gap, flexion gap width, and limb alignment was undertaken before and after the posterior condylar osteophyte removal. This study reveals that the mean of the size of posterior condylar osteophyte after removal is 8.96 mm. The posterior condylar osteophyte has an effect on the increasing of medial extension gap and lateral extension in average 0.74 ± 0.72 mm. and 0.42 ± 0.67 mm. respectively. It also increases 0.71 ± 1.00 mm. in medial flexion gap and 0.97 ± 1.47 mm. in lateral flexion gap. After the posterior condylar osteophyte removal the mean of varus deformity is decreased 0.90° ± 1.14 ° while the mean of extension angle of sagittal limb alignment is increased 1.61°±1.69°. There is also a significant relationship between the size of posterior condylar osteophyte and the increasing of lateral flexion gap and also with the varus deformity decreasing. If the size of posterior condylar osteophyte is increased 10 mm. the lateral flexion gap will be increased 1.15 mm. and varus deformity will be decreased 0.75 degree

Aims. Medial unicompartmental knee arthroplasty (UKA) is undertaken in patients with a passively correctable varus deformity. Our hypothesis was that restoration of natural soft tissue tension would result in a comparable lower limb alignment with the contralateral normal lower limb after mobile-bearing medial UKA. Patients and Methods. In this retrospective study, hip-knee-ankle (HKA) angle, position of the weight-bearing axis (WBA) and knee joint line obliquity (KJLO) after mobile-bearing medial UKA was compared with the normal (clinically and radiologically) contralateral lower limb in 123 patients. Results. Postoperatively, HKA angle was restored to within ±3° of the contralateral lower limb in 87% of patients andWBA passed within ±1 Kennedy and White's tibial zone of the contralateral normal lower limb in 95% of patients. The difference in the mean KJLO between the two groups was not significant (p=0.05) and the KJLO was within ±3° of the contralateral normal lower limb in 96% of patients. Conclusion. Lower limb alignment & knee joint line obliquity after mobile-bearing medial UKA were comparable to the unaffected contralateral limb in most patients. Clinical Relevance. Comparison with the contralateral normal lower limb is a reliable method to evaluate and validate limb mechanical alignment after mobile-bearing medial UKA

Background. The optimal reference for rotational positioning of femoral component in total knee replacement (TKR) is debated. Navigation has been suggested for intra-op acquisition of patient's specific kinematics and functional flexion axis (FFA). Questions/Purposes. To prospectively investigate whether pre-operative FFA in patients with osteoarthritis (OA) and varus alignment changes after TKR and whether a correlation exists between post-op FFA and pre-op alignment. Patients and Methods. A navigated TKR was performed in 108 patients using a specific software to acquire passive joint kinematics before and after TKR. The knee was cycled through three passive range of motions (PROM), from 0° to 120°. FFA was computed using the mean helical axis algorithm. The angle between FFA and surgical TEA was determined on frontal (α. f. ) and axial (α. a. ) plane. The pre- and post-op hip-knee-ankle angle (HKA) was determined. Results. Post-op FFA was different from pre-op FFA only on frontal plane. No significant difference was found on axial plane. No correlation was found between HKA-pre and α. A. -pre. A significant correlation was found between HKA-pre and α. F. –pre. Conclusions. TKR modifies FFA only on frontal plane. No difference was found on axial plane. Pre-op FFA is in a more varus position respect to TEA. The position of FFA on frontal plane is dependent on limb alignment. TKR modifies the position of FFA only on frontal plane. The position of FFA on axial plane is not dependent on the amount of varus deformity and is not influenced by TKR

BACKGROUND:. The optimal reference for rotational positioning of femoral component in total knee replacement (TKR) is debated. Navigation has been suggested for intra-op acquisition of patient's specific kinematics and functional flexion axis (FFA). QUESTIONS/PURPOSES:. To prospectively investigate whether pre-operative FFA in patients with osteoarthritis (OA) and varus alignment changes after TKR and whether a correlation exists between post-op FFA and pre-op alignment. PATIENTS AND METHODS:. A navigated TKR was performed in 108 patients using a specific software to acquire passive joint kinematics before and after TKR. The knee was cycled through three passive range of motions (PROM), from 0° to 120°. FFA was computed using the mean helical axis algorithm. The angle between FFA and surgical TEA was determined on frontal (αf) and axial (αa12) plane. The pre- and post-op hip-knee-ankle angle (HKA) was determined. RESULTS:. Post-op FFA was different from pre-op FFA only on frontal plane. No significant difference was found on axial plane. No correlation was found between HKA-pre and αA-pre. A significant correlation was found between HKA-pre and αF–pre. CONCLUSIONS:. TKR modifies FFA only on frontal plane. No difference was found on axial plane. Pre-op FFA is in a more varus position respect to TEA. The position of FFA on frontal plane is dependent on limb alignment. TKR modifies the position of FFA only on frontal plane. The position of FFA on axial plane is not dependent on the amount of varus deformity and is not influenced by TKR

Twenty patients underwent simultaneous bilateral medial unicompartmental knee arthroplasty. Pre-operative hip-knee-ankle alignment and valgus stress radiographs were used to plan the desired post-operative alignment of the limb in accordance with established principles for unicompartmental arthroplasty. In each patient the planned alignment was the same for both knees. Overall, the mean planned post-operative alignment was to 2.3° of varus (0° to 5°).

The side and starting order of surgery were randomised, using conventional instrumentation for one knee and computer-assisted surgery for the opposite side.

The mean variation between the pre-operative plan and the achieved correction in the navigated and the non-navigated limb was 0.9° (sd 1.1; 0° to 4°) and 2.8° (sd 1.4; 1° to 7°), respectively. Using the Wilcoxon signed rank test, we found the difference in variation statistically significant (p < 0.001).

Assessment of lower limb alignment in the non-navigated group revealed that 12 (60%) were within ± 2° of the pre-operative plan, compared to 17 (87%) of the navigated cases.

Computer-assisted surgery significantly improves the post-operative alignment of medial unicompartmental knee arthroplasty compared to conventional techniques in patients undergoing bilateral simultaneous arthroplasty. Improved alignment after arthroplasty is associated with better function and increased longevity.

Abstract

Background

A principle of Total Knee Arthroplasty (TKA) is to achieve a neutral standing coronal alignment of the limb (Hip Knee Ankle (HKA) angle) to reduce risks of implant loosening, reduce polyethylene wear, and optimise patella tracking. Several long-term studies have questioned this because the relationship between alignment and implant survivorship is weaker than previously reported. We hypothesize standing HKA poorly predicts implant failure because it does not predict dynamic HKA, dynamic adduction moment, and loading of the knee during gait. Therefore, the aim of our study is to assess the relationship between the standing (or static) and the dynamic (gait activity) HKAs.

A principle of Total Knee Arthroplasty (TKA) is to achieve a neutral standing coronal alignment of the limb (Hip Knee Ankle (HKA) angle) to reduce risks of implant loosening, reduce polyethylene wear, and optimise patella tracking. Several long-term studies have questioned this because the relationship between alignment and implant survivorship is weaker than previously reported. We hypothesize standing HKA poorly predicts implant failure because it does not predict dynamic HKA, dynamic adduction moment, and loading of the knee during gait. Therefore, the aim of our study is to assess the relationship between the standing (or static) and the dynamic (gait activity) HKAs.

We performed a prospective study on a cohort of 35 patients (35 knees) who were treated with a posterior-stabilized TKA for primary osteoarthritis between November 2012 and January 2013. Three months after surgery each patient had a standardized digital full-leg coronal radiographs and was classified as neutrally aligned TKA (17 patients), varus aligned (9 patients), and valgus aligned (4 patients). Patients then performed a gait analysis for level walking and dynamic HKA and adduction moment during the stance phase of gait were measured.

We found standing HKA having a moderate correlation with the peak dynamic varus (r=0.318, p=0.001) and the mean and peak adduction moments (r=0.31 and r=-0.352 respectively). In contrast we did not find a significant correlation between standing HKA and the mean dynamic coronal alignment (r=0.14, p=0.449). No significant differences were found for dynamic frontal parameters (dynamic HKA and adduction moment) between patients defined as neutrally aligned or varus aligned.

In our practice, the standing HKA after TKA was of little value to predict dynamic behaviour of the limb during gait. These results may explain why standing coronal alignment after TKA may have limited influence on long term implant fixation and wear.

Bilateral sequential total knee replacement with a Zimmer NexGen prosthesis (Zimmer, Warsaw, Indiana) was carried out in 30 patients. One knee was replaced using a robotic-assisted implantation (ROBOT side) and the other conventionally manual implantation (CON side). There were 30 women with a mean age of 67.8 years (50 to 80).

Pre-operative and post-operative scores were obtained for all patients using the Knee Society (KSS) and The Hospital for Special Surgery (HSS) systems. Full-length standing anteroposterior radiographs, including the femoral head and ankle, and lateral and skyline patellar views were taken pre- and post-operatively and were assessed for the mechanical axis and the position of the components. The mean follow-up was 2.3 years (2 to 3).

The operating and tourniquet times were longer in the ROBOT side (p < 0.001). There were no significant pre- or post-operative differences between the knee scores of the two groups (p = 0.288 and p = 0.429, respectively). Mean mechanical axes were not significantly different in the two groups (p = 0.815). However, there were more outliers in the CON side (8) than in the ROBOT side (1) (p = 0.013). In the coronal alignment of the femoral component, the CON side (8) had more outliers than the ROBOT side (1) (p = 0.013) and the CON side (3) also had more outliers than the ROBOT side (0) in the sagittal alignment of the femoral component (p = 0.043). In terms of outliers for coronal and sagittal tibial alignment, the CON side (1 and 4) had more outliers than the ROBOT side (0 and 2).

In this series robotic-assisted total knee replacement resulted in more accurate orientation and alignment of the components than that achieved by conventional total knee replacement.

Many studies have already been published to prove the improved accuracy in achieving the ideal post-operative long leg alignment when using computer navigation in total knee arthroplasty (TKA). Surgeons who use traditional instrumentation with a fixed distal femoral resection angle (most commonly 6°) assume little or no variation in the angle between the anatomical and mechanical axis of the femur (FMA angle) in different patients.

The aims of this study were to investigate the distribution of the FMA angle in pathological knees of patients about to undergo TKA and to analyse if there was any correlation between the FMA angle and the pre-operative lower limb alignment in the coronal plane (varus or valgus).

The study consisted of 158 consecutive patients undergoing 174 primary TKA between January and October 2007. All patients had pre-operative digital Hip-Knee-Ankle radiographs. The FMA angle and the mechanical femorotibial angle (MFT angle) were measured in all cases. Intra- and inter-observer variation was measured by second observer readings and repeated measurements.

The mean age of the study cohort was 69.9 years (SD 8.7 years). There were 75 male and 99 female knees. The repeatability for measurement of the FMA angle was good (intra-observer Intra Correlation Coefficient (ICC) = 0.91, inter-observer ICC = 0.85) and for the measurement of MFT angle was very good (intra-observer ICC = 0.99, inter-observer ICC = 0.99). There were 135 knees with a varus or neutral alignment and 39 knees with valgus alignment. The median alignment was 6.5° varus ranging from 23° varus to 16° valgus. The FMA angle was between 2° and 9°, with a median of 6°. The FMA angle was 6° in 35.4% of cases, 5° in 22.9% and 7° in 18.3%. There was a statistical significant correlation between the FMA angle and the pre-operative lower limb alignment (Pearson correlation coefficient = −0.5, p < 0.001), with valgus knees having on average a lower FMA angle. The group of females and males had statistically different FMA angles (Mann-Whitney, p < 0.001) with females having on average a lower FMA angle. Cluster analysis based on the original clinical definitions of severe varus, varus and valgus gave three groups of FMA angle for MFT angle < 8° varus, MFT angle of 8° varus to 1° valgus and MFT angle > 1° valgus. There was a statistically significant difference in median FMA angle between these three groups (Kruskal-Wallis, p < 0.001).

This study indicates that one of the main reasons why optimal post-operative coronal alignment cannot be achieved with a fixed distal femoral resection angle is the fact that the FMA angle has a wide, natural distribution. It is possible that better results may be achieved with traditional instrumentation by individual measurement of FMA angle for each patient pre-operatively and adjusting the distal femoral resection to account for this. However, with computer navigation the distal femoral cut is adjusted for each patient.