Tibial Mechanical Axis Research Articles

Radiological Outcomes and Operative Time following Total Knee Arthroplasty using Accelerometer-based, Portable Navigation versus Conventional Inter-Medullary Alignment Guides

Objectives:Accelerometer-based, portable navigation instrumentation is a new method of achieving desired resection alignments in total knee arthroplasty (TKA).Methods:After randomisation and the application of exclusion criteria, 79 knees were analysed. 42 patients which underwent TKA using conventional intra-medullary (IM) alignment guides were compared to 37 patients with the use of accelerometer-based, portable navigation device (KneeAlign; OrthoAlign Inc, Aliso Viejo, California). Radiographic results were obtained from post-operative computer-tomography following the CT Perth Protocol.Results:In the IM cohort, 81.0% of patients had a coronal alignment within 3° of a neutral mechanical axis (vs 83.8% with KneeAlign, p=0.74), 81.0% had a femoral coronal alignment within 2° of perpendicular to the femoral mechanical axis (vs 89.2% with KneeAlign, p=0.31), and 92.9% had a tibial coronal alignment within 2° of perpendicular to the tibial mechanical axis (vs 81.1% with KneeAlign, p=0.12). Regarding sagittal alignment, the IM cohort had 90.5% of patients with femoral component alignment within 2° of optimum (vs 91.9% with KneeAlign, p=0.83) and 92.9% had a tibial component alignment within 2° of the optimal tibial slope (vs 89.2% with KneeAlign, p=0.57). The mean tourniquet time (from incision to completion of coronal bone resections) in the IM cohort was 16.5± 8.9 minutes vs 22.2 ± 7.6 minutes in the KneeAlign cohort (p<0.003).Conclusion:Accelerometer-based, portable navigation has a statistically similar outcome in alignment following TKA as IM guides. It is noted that using the portable navigation device does prolong surgical time compared to conventional IM surgery and this may be due to the learning curve.

No condylar lift-off occurs because of excessive lateral soft tissue laxity in neutrally aligned total knee arthroplasty: a computer simulation study.

Condylar lift-off can induce excessive polyethylene wear after total knee arthroplasty (TKA). A computer simulation was used to evaluate the influence of femoral varus alignment and lateral collateral ligament (LCL) laxity on lift-off after single-design TKA. It was hypothesised that proper ligament balancing and coronal alignment would prevent lift-off. The computer model in this study is a dynamic musculoskeletal program that simulates gait up to 60° of knee flexion. The lift-off phenomenon was defined as positive with an intercomponent distance of >2mm. In neutrally aligned components in the coronal plane, the femoral and tibial components were set perpendicular to the femoral and tibial mechanical axis, respectively. The femoral coronal alignment was changed from neutral to 5° varus in 1° increments. Simultaneously, the LCL length was elongated from 0 to 5mm in 1-mm increments to provide a model of pathological slack. Within 2° of femoral varus alignment, lift-off did not occur even if the LCL was elongated by up to 5mm. However, lift-off occurred easily in the stance phase in femoral varus alignments of >3° with slight LCL slack. The contact forces of the tibiofemoral joint were influenced more by femoral varus alignment than by LCL laxity. Aiming for neutral alignment in severely varus knees makes it difficult to achieve appropriate ligament balance. Our study suggests that no lift-off occurs with excessive LCL laxity alone in a neutrally aligned TKA and therefore that varus alignment should be avoided to decrease lift-off after TKA. Case series, Level IV.

Tibial Stems in Revision Total Knee Arthroplasty: Is There an Anatomic Conflict?

Tibial stems are frequently used in revision total knee arthroplasty (TKA). We investigated the effect of tibial stems on final component alignment and tray position. Thirty 3D reconstructed cadaveric tibial models were classified according to canal bow angle. After virtual implantation of 120 mm and 200 mm stemmed tibial components, deviation from native mechanical axis was measured. Tibial alignment valgus malposition of up to three degrees occurred and most pronounced with 120 mm stems. Canal alignment using 200 mm stems deviated tray position medially and posteriorly. Mild to moderate valgus bowing of the tibial canal is not uncommon (57% in our series) possibly leading to valgus malalignment. Anatomic conflict between the tibial mechanical axis and intramedullary canal can exist, with alignment and tray placement implications.

Computer-assisted total knee replacement after medial opening wedge high tibial osteotomy: medium-term results in a series of ninety cases.

The aim of this study was to evaluate the medium-term results of total knee replacements (TKRs) after medial opening wedge valgising tibial osteotomy to those of primary TKR (TKR1). The hypothesis being that there would be no difference in results between these groups. Series 1 was made up of 45 TKRs after medial opening wedge high tibial osteotomy (MHTO)-30 men and ten women at an average age of 69 ± seven years (54-82). This was compared to a second series of TKR1s-30 men and ten women at an average age of 69 ± seven years (55-78). The average IKS scores were 91 ± 22.5 points (42-129) and 86 ± 18 points (38-116) in the two groups respectively. The average pre-operative HKA angle was 179 ± 5° (169-193°) in group 1, and 173 ± 7.5° (161-193°) in group 2. Tibial mechanical axes were 90.5 ± 4° (of which 24 knees had a valgus angle) and 85.05 ± 3.5° (79-93°) in the two groups respectively. All patients were reviewed at an average follow-up of 47 ± 24.5 months for series 1 and 185 ± 8.5 months for series 2. The average IKS score was 184 ± 6 for series 1 (172-200) and 185 ± 8.5 (163-200) for series 2 (p = 0.872). Thirty-seven patients in series 1 and 38 patients in series 2 were either extremely satisfied or satisfied with the intervention. The average post-operative HKA angle was 180.5 ± 2.5° and 181 ± 2° (p = 0.122) and the average tibial mechanical axis was 89 ± 1.5° against 90 ± 1° (p = 0.001). The results of the 24 knees with a valgus tibial mechanical axis were statistically no different. TKRs post medial opening wedge high tibial osteotomy have identical results to primary TKRs even in knees with a valgus tibial mechanical axis.

La largeur pelvienne influence-t-elle la course patellaire ? Une étude radiographique comparative entre les sexes

IntroductionPelvic width has been believed to affect patellar tracking by influencing the quadriceps angle (Q-angle). Anatomically, the upper arm of the Q-angle does not closely match the orientation of the quadriceps femoris. The pelvis is often considered wider and the Q-angle larger in female than in male individuals. The purpose of this retrospective study was to investigate the accuracy of such an assumption by using a radiologic comparison, which might be more objective. Materials and methodsOne hundred consecutive adult patients (50 men and 50 women) aged 18–30 years with unilateral injury to the lower extremity were studied. Full-length standing X-rays of these patients were used to analyze the relationship between the pelvis and the uninjured lower extremity and compare it between the sexes. The pelvic width was defined as the distance between the centers of the bilateral femoral heads. ResultsThe pelvic width did not differ statistically between male and female (P=0.74). The femur length and sum of the lengths of the femur and tibia differed between the sexes (both P<0.001). Normalization of the pelvic width to the femur length or sum of the lengths of the femur and tibia resulted in a significant difference between male and female (P<0.001). The angle formed by the femoral and tibial mechanical axes correlated strongly with the angle formed by the femoral anatomic and tibial mechanical axes (Pearson correlation coefficient=0.89). DiscussionPelvic width does not differ with respect to gender. The pelvis may appear relatively wider in women due to the difference in body height. However, this difference may not increase Q-angle. Patellar mal-tracking may stem from other, more critical predisposing factors. Level of evidenceLevel IV. Anatomic study.

Functional joint line obliquity after kinematic total knee arthroplasty.

Kinematic total knee arthroplasty (TKA) is an emerging technique, but concerns remain around the effect of implanting the prosthesis in more anatomic orientations. Native knees show variation in joint line orientation relative to the tibial mechanical axis but the joint line remains parallel to the floor when standing. This study was undertaken to evaluate joint line obliquity relative to the floor when weight-bearing after kinematic TKA to see if a similar effect occurs. Preoperative and postoperative measurements were taken for 55 consecutive kinematically aligned TKAs, including the joint line orientation angle (JLOA), formed between the joint line and a line parallel to the floor. The mean medial proximal tibial angle (MPTA) was 3.4° varus pre-operatively (1.7° valgus to 7.9° varus, SD 2.0), and 3.0° varus postoperatively (5.5° valgus to 6.5° varus, SD 2.1). The mean postoperative JLOA was 1.0° varus with a smaller range than the MPTA (2.6° valgus to 6° varus, SD 1.9). The difference between these two measurements was significant (mean 2°, SD 2.5, p < 0.001). Relative to the mechanical axis, 33 tibial components would be considered at risk outliers, being orientated at more than 3° in varus or valgus. However, only six components were outside this range relative to the vertical, all in varus (mean 4.2°). This latter measurement may better represent how the prosthesis is functionally loaded and is similar to mechanically aligned TKAs with good survivorship. This may help explain why kinematic alignment does not lead to higher earlier failure rates that may result if similar orientations were seen with mechanically aligned TKA.

Does pelvic width influence patellar tracking? A radiological comparison between sexes

IntroductionPelvic width has been believed to affect patellar tracking by influencing the quadriceps angle (Q-angle). Anatomically, the upper arm of the Q-angle does not closely match the orientation of the quadriceps femoris. The pelvis is often considered wider and the Q-angle larger in female than in male individuals. The purpose of this retrospective study was to investigate the accuracy of such an assumption by using a radiologic comparison, which might be more objective. Materials and methodsOne hundred consecutive adult patients (50 men and 50 women) aged 18–30 years with unilateral injury to the lower extremity were studied. Full-length standing X-rays of these patients was used to analyze the relationship between the pelvis and the uninjured lower extremity and compare it between the sexes. The pelvic width was defined as the distance between the centers of the bilateral femoral heads. ResultsThe pelvic width did not differ statistically between male and female (P=0.74). The femur length and sum of the lengths of the femur and tibia differed between the sexes (both P<0.001). Normalization of the pelvic width to the femur length or sum of the lengths of the femur and tibia resulted in a significant difference between male and female (P<0.001). The angle formed by the femoral and tibial mechanical axes correlated strongly with the angle formed by the femoral anatomic and tibial mechanical axes (Pearson correlation coefficient=0.89). DiscussionPelvic width does not differ with respect to gender. The pelvis may appear relatively wider in women due to the difference in body height. However, this difference may not increase Q-angle. Patellar mal-tracking may stem from other, more critical predisposing factors. Level of evidenceLevel IV. Anatomic study.

Clinical faceoff: Neutrally versus kinematically aligned TKA.

While the number of primary TKAs completed in the United States is expected to increase dramatically in the years to come, approximately 20% of patients undergoing primary TKA are dissatisfied with the procedure [5, 11]. As such, substantial healthcare resources have been devoted to the development and use of patient-specific instrumentation (PSI), computer navigation, robotics, and other tools that help achieve a neutrally aligned TKA in the coronal plane [1, 2, 14]. On the other hand, some investigators support a different approach: The concept of a kinematically aligned TKA [6, 8–10]. In general terms, a neutrally aligned TKA cuts the distal femur and proximal tibia 90° to the respective mechanical axes, while a kinematically aligned TKA incorporates 3° of varus into the tibia and an additional 3° of valgus into the femur. However, there remains considerable controversy in this area, in large measure because of a relative paucity of research comparing the approaches. As such, I have invited two internationally known experts to discuss these two approaches to obtaining a well-aligned TKA – the neutrally aligned and kinematically aligned techniques.

Proximal tibial resection in varus-deformed tibiae during total knee arthroplasty: an in vitro study using sawbone model.

This study was designed to investigate accuracy of the tibial component in the coronal plane when the lateral condylar eminence was determined as the proximal reference of proximal tibial cutting in varus-deformed knees. Varus-deformed tibiae were designed using the entire tibia sawbone models of four categories (varus 0°, 5°, 10° and 15°). Each of ten sawbones was allocated into four in each group. All sawbones were osteotomised with the proximal reference of proximal tibial cutting as the lateral intercondylar eminence. The thickness of the cut tibial medial and lateral condyles were measured. After implantation of the tibial component, anteroposterior (AP) radiographs were obtained. An independent examiner evaluated angles between the perpendicular line to the tibial tray and the shaft/mechanical axes of the tibia. The mean angular difference between the axis perpendicular to the prosthesis and the mechanical/shaft axes of the tibia were not significant in any group. When the tibial component coronal alignment was measured based on the tibial mechanical axis, no significant difference was observed in any group, but when based on the shaft axis of the tibia, it was significantly different between Group A and Group D. Accuracy of the tibial component alignment was acceptable in varus-deformed knees when the lateral intercondylar eminence was used as a reference for tibial osteotomy, although any prolonged clinical benefits will require long-term in vivo study.

Fibular axes are not a reliable landmark for tibial mechanical axes of osteoarthritic knees that underwent total knee arthroplasty.

The sagittal fibular axis serves as an intra-operative landmark during conventional total knee arthroplasty (TKA); however, only a few relevant anatomical studies have been published regarding its use as an extramedullary guide. Furthermore, the correlation between the coronal fibular and tibial mechanical axes in osteoarthritic knees has been only reported once. Here, the hypothesis of this study is that the fibula can be a reliable intra-operative landmark, in the sagittal and coronal planes, among patients with osteoarthritis who have undergone TKA. Osteoarthritic knees (n = 62) after TKA were evaluated using three-dimensional image-matching software. The angles between the tibial mechanical axis and the fibular shaft axis were measured in the sagittal and coronal planes. Moreover, correlations between the angles and patient-specific factors were evaluated. The mean angle between the tibial mechanical and fibular shaft axes was 2.6° ± 2.3° for posterior inclination in the sagittal plane and 0.9° ± 2.0° for varus inclination in the coronal plane. The percentage of subjects with the fibular shaft axis within 2° of the tibial mechanical axis was 17.7 and 69.3 % in the sagittal and coronal planes, respectively. No patient-specific factors were correlated with the angle between the tibial mechanical and fibular shaft axes. The angle between the tibial mechanical and fibular shaft axes differed among patients, independent of patient-specific factors, and did not appear to be a reliable intra-operative landmark. Surgeons should use values from individual pre-operative evaluations of the axis as reference for conventional TKA. Case series with no comparison group, Level IV.

Extreme Variability in Posterior Slope of the Proximal Tibia: Measurements on 2395 CT Scans of Patients Undergoing UKA?

Data regarding the posterior slope of the tibia (PTS) are limited and sometimes conflicting. The purpose of this study was to determine the native posterior tibial slope in patients undergoing a medial or lateral UKA. A retrospective review was performed on 2395 CT scans in patients indicated for UKA, and the PTS of the osteoarthritic compartment was measured relative to a plane set perpendicular to the sagittal, tibial mechanical axis. The mean preoperative PTS in patients undergoing medial UKA was 6.8°+3.3°, with 34.3% between 4° and 7°. The mean preoperative PTS in patients undergoing lateral UKA was 8.0°+3.3°, with 27.5% between 4° and 7°. If attempting to recreate a patient's preoperative tibial slope, a routine target of 5° to 7° will produce a posterior slope less than the patient's native anatomy in 47% of patients undergoing UKA. This is the first, large CT-based review of posterior slope variation of the proximal tibia in patients undergoing UKA.

Accuracy of bony landmarks for restoring the natural joint line in revision knee surgery: an MRI study

Restoring the joint line (JL) improves clinical and functional outcome in total knee arthroplasty (TKA). Therefore, anatomical landmarks to approximate the JL have been published. So far, the natural deviation of the JL 90° to the mechanical tibial axis has not been considered. Thus, we designed this study to: (1) determine the natural JL of knees in healthy persons in respect to the mechanical tibial axis, (2) validate and double-check intra-operative bony landmarks already been published in respect to the natural JL and (3) find possible correlations between distances from bony landmarks to the JL and femoral and tibial width. Eighty MRI scans of knees of healthy persons were examined by two independent observers. Distances from the tip of the fibular head (FH), the medial (ME) and lateral (LE) epicondyles and the adductor tubercle (AT) to the JL within the medial and lateral compartment were measured. Further, we determined the orientation of the JL in respect to the mechanical axis of the tibia. Interobserver correlations were calculated. Differences were analyzed using Student's t test. Linear regression models were calculated to analyze correlations. Interobserver correlation was excellent. Mean JL deviation was 4.2° varus. Distance between the FH, ME, LE and AT to the JL within the medial compartment was 12.2, 33.9, 33.4 and 45.4 mm, respectively. Within in the lateral compartment, distances were 15.3, 31.0, 30.6 and 42.3 mm to the JL. Strong correlation was found between femoral width and distances from the AT, ME and LE to the JL. In TKA, the JL is usually altered due to the classic resection technique, which does not respect the natural deviation of the JL. Estimating the natural JL by adding absolute values to bony landmarks, as proposed in the literature, is not recommended. According to our data, the JL can be best estimated by adding the calculated value: 6.40 + (width femur [mm] × 0.49) to the AT.

Intramedullary control of distal femoral resection results in precise coronal alignment in TKA

There is still a relevant rate of outliers in coronal alignment >3° when the conventional technique is used, potentially accompanied by a poorer long-term clinical outcome and a reduced longevity of the implant. Intraoperative implementation of preoperative planning and above all checking of the bone resections carried out are decisive for reinstating a straight leg axis. Intramedullary control of femoral resection has not been described to date. The objective of this study was to present a new technique for the intramedullary control of femoral resection and the results obtained using this method. All patients who underwent primary total knee arthroplasty with the new intramedullary control of femoral resection were included in this retrospective study. The frequency of the need for correction of the saw cuts was documented. The radiological assessment included pre- and postoperative whole-leg standing radiographs. In the process, the whole-leg axis, AMA, entry point, LDFA and MPTA were evaluated preoperatively. On the postoperative radiographs, the whole-leg axis and the alignment of the femoral and tibial components were evaluated. One hundred and sixty-two total knee arthroplasties (TKAs) were included in the study. The average age was 68.7 years. The preoperative malalignment was on the average 8.2° ± 4.7° (23.8° varus to 17.3° valgus). The postoperative whole-leg axis was on the average 1.3° ± 1.1° (5.5° varus to 4.3° valgus). The femoral component showed a deviation from the mechanical axis of 0.1° ± 1.2° (4.3° varus to 3.7° valgus) and the tibial component a deviation from the mechanical tibial axis of 0.3° ± 1.2° (4.2° varus to 2.5° valgus). The new technique of intramedullary control of distal femoral resection, together with preoperative planning, leads to a precise alignment of the femoral component in the coronal plane. Thus, for the first time, a simple and effective tool for checking distal femoral resection is available for standardized use.

Collateral soft tissue release in primary total knee replacement

The aim of this study was to assess the rate of collateral soft tissue release required in navigated total knee arthroplasty (TKA) to achieve an intra-operative coronal femoral tibial mechanical axis (FTMA) in extension of 0 ± 2°. The primary outcomes assessed were post-operative coronal plane alignment and rate of collateral soft tissue release. The secondary outcomes were range of motion, function, patient satisfaction, and complication rates at one-year follow-up. This is a prospective study of 224 knees. No exclusions were made on the basis of pathology or severity of deformity. Pre-operative FTMA ranged from 27° valgus to 25° varus (mean: −4.5° SD 7.6). Soft tissue release was carried out in 5 of 224 knees (2.2%). Post-operative weight-bearing radiological FTMA ranged from 7° valgus to 8° varus (mean: −0.4° SD 2.5°). Two hundred and ten knees (96%) were within 0 ± 5° of neutral. At one year, median maximum flexion was 100° (IQR 15°) and extension was 0°; mean post-operative Oxford Knee Score had improved from 42 to 23; and 91% of patients were satisfied or very satisfied, with only 2% being dissatisfied. We have found that in the vast majority of cases, including those with large pre-operative coronal deformity in extension, good outcomes in terms of coronal alignment, range of movement, function and patient satisfaction can be achieved.

Anatomical references for tibial sagittal alignment in total knee arthroplasty: A comparison of three anatomical axes based on 3D reconstructed CT images

Background This study was designed to analyze three tibial axis reference lines including the anterior tibial cortex (ATC) line, the fibular line (FL), and the anatomical axis of tibia (AAT) line, to determine which line most closely parallels the mechanical axis (MA) of the tibia in the sagittal plane. The clinical relevance of the study is that through finding a reliable landmark on the leg, a surgeon may minimize posterior tibial slope measurement errors thereby and improving the technique for assuring proper alignment of total knee arthroplasty. Methods The material for this study included CT scans of the tibia from 85 consecutive patients and 168 knees (78 without osteoarthritis (OA) and 90 knees with OA). Measurements of the angles between the tibial mechanical axis and each of three reference lines in the sagittal plane were carried out using 3D imaging software. Results Mean angles of 168 knees were as follows: aMT (3.96±0.85)°, aMF (0.70±0.58)°, and aMA (1.40±0.66)°, (aMT: an angle between MA and ATC, aMF: an angle between MA and FL, aMA: an angle between MA and AAT. All above-mentioned angles were measured in the sagittal plane of tibia) and the aMF was significantly smaller than the others (P &lt;0.0001). The mean value of the medial tibial slope angle vs. the MA was (9.19±3.97)°, and this was significantly larger than the mean lateral slope angle of (6.62±4.23)° (P &lt;0.0001). The difference between aMF without OA and with OA was not statistically significant (P=0.5015) and the association between the aMT and aMA was strong (r=0.82, P &lt;0.01). Conclusions FL was more closely parallel to the MA of tibia, and more showed less variation between OA and non-OA controls than ATC and AAT lines. Furthermore, the amount of posterior slope in medial plateau was greater than that in lateral plateau. The findings of this analysis suggest that when using the anterior tibial cortex line as is commonly done with extramedullary tibial resection guides, the tibial resection should be sloped approximately four degrees more posteriorly.

The role of the medial ligaments in lateral stabilization of the ankle joint: an in vitro study.

The deltoid ligament complex is known as medial stabilizer in the ankle against pronation/eversion. Lateral dual-ligament laxity often results in chronic ankle instability with recurring ankle sprain trauma. The goal of this study is to examine the lateral stabilizing role of the deltoid ligament complex against supination/inversion in case of existing lateral ligament instability. A torsion simulation was performed on 12 fresh human lower leg cadaver specimens in a loading frame and a specially designed mounting platform. The preset torsion between tibia and calcaneus was primarily set at 30° of internal rotation on specimen in plantar flexion and hindfoot inversion. The measured variable was the resisting torque recorded around mechanical tibial axis, which ensures stability in ankle sprain trauma. The first series of measurements were performed on healthy specimens and the following after transecting structures in following order: anterior talofibular ligament (ATFL) in combination with calcaneofibular ligament (CFL), followed by anterior tibiotalar ligament and posterior tibiotalar ligament and finally tibiocalcaneal ligament (TCL). The combined lateral ATFL and CFL instability showed a decrease in the resisting torque, which ensures stability in ankle sprain trauma. Only a transection of TCL (superficial layer of deltoid ligament complex) with existing lateral dual-ligament instability results in a significant decrease in torque (p<0.0001). The goal of the study was to provide the orthopaedic and/or trauma surgeon with quantitative data that may be referred to the substantial stabilizing effect of TCL against supination/inversion in the ankle joint in case of repetitive sprain trauma at a present lateral ligament lesion. Diagnostics of and treatment for lateral ligament instability need to consider the deltoid ligament complex,especially TCL in clinical routine.

Extramedullary Guides Versus Portable, Accelerometer-Based Navigation for Tibial Alignment in Total Knee Arthroplasty: A Randomized, Controlled Trial: Winner of the 2013 HAP PAUL Award

Extramedullary (EM) tibial alignment guides have demonstrated a limited degree of accuracy in total knee arthroplasty (TKA). The purpose of this study was to compare the tibial component alignment obtained using a portable, accelerometer-based navigation device versus EM alignment guides. One hundred patients were enrolled in this prospective, randomized controlled study to receive a TKA using either the navigation device, or an EM guide. Standing AP hip-to-ankle and lateral knee-to-ankle radiographs were obtained at the first, postoperative visit. 95.7% of tibial components in the navigation cohort were within 2° of perpendicular to the tibial mechanical axis, versus 68.1% in the EM cohort (P<0.001). 95.0% of tibial components in the navigation cohort were within 2° of a 3° posterior slope, versus 72.1% in the EM cohort (P=0.007). A portable, accelerometer-based navigation device decreases outliers in tibial component alignment compared to conventional, EM alignment guides in TKA.

Effects of tibial torsion on distal alignment of extramedullary instrumentation in total knee arthroplasty

Background and purpose Whether tibial torsion affects the positioning of extramedullary instrumentation and is a possible factor in malalignment of the tibial component in total knee arthroplasty (TKA) is unknown. We assessed the influence of tibial torsion on distal alignment of extramedullary systems for TKA, using the center of the intermalleolar distance as anatomical reference at the ankle joint.Patients and methods We analyzed CT scans of knee and ankle joints of 50 patients with knee osteoarthritis (mean age 73 years, 52 legs). The tibial mechanical axis was identified and translated anteriorly at the level of the medial one-third (proximal AP axis 1), at the medial border of the tibial tuberosity (proximal AP axis 2), and at the level of the talar dome (distal AP axis). The center of the intermalleolar distance and the width of the medial and lateral malleolus were calculated. The proximal AP axes 1 and 2 were translated at the level of the ankle joint and any difference between their alignment and the distal AP axis was calculated as angular and linear values.Results The center of the ankle joint was located, on average 2 mm medial to that of the intermalleolar distance. The distal AP axis was externally rotated by 18° and 27° compared to the proximal AP axes 1 and 2, respectively. Overall, the center of the ankle joint was shifted laterally by 9–11 mm with respect to the proximal AP tibial axes.Interpretation To avoid a varus tibial cut in TKA, extramedullary alignment systems should be aligned more medially at the ankle joint than previously thought, due to the effect of tibial torsion and—to a lesser extent—to the different malleolar width.

Cylindrical Axis, Not Epicondyles, Approximates Perpendicular to Knee Axes

The transepicondylar axis (TEA) is often used as a surrogate for the flexion-extension axis, ie, the axis around which the tibia moves in space, because of a belief that both axes lie perpendicular to the mechanical axis. However, studies suggest the cylindrical axis (CA), defined as a line equidistant from contact points on the medial and lateral condylar surfaces from 10(o) to 120(o) flexion, more closely approximates the axis around which the tibia moves in space. We examined the TEA and CA angles relative to mechanical axes to determine whether one more consistently and closely approximates the surgical goal of orthogonality to the mechanical axis. Three-dimensional (3-D) models were reconstructed from CT scans of five cadaver limbs. Three observers repeated three measurement sets to locate the TEA, CA, and femoral mechanical and tibial mechanical axes. Angles of the TEA and CA relative to the mechanical axes were calculated in two-dimensions (2-D) and as 3-D projections and compared for differences in magnitude and variance. Angles between CA and the mechanical axes were closer to 90° than the TEA in 2-D (92° versus 94° for the femur, 93° versus 94° for the tibia) and 3-D (88° versus 87° for the femur, 88° versus 86° for the tibia). Variance of the TEA was higher than the CA in 2-D. The CA forms angles more orthogonal to the mechanical axes of the thigh and leg than the TEA. Although we found a consistently greater deviation of the TEA from the mechanical axis than the CA with small differences, future studies will need to determine whether these differences are biomechanically or clinically important.

Patient specific cutting guides versus an imageless, computer-assisted surgery system in total knee arthroplasty

BackgroundPatient specific cutting guides (PSC) in total knee arthroplasty (TKA) have recently been introduced, in which preoperative 3-dimensional imaging is used to manufacture disposable cutting blocks specific to a patient's anatomy. The purpose of this study was to compare the alignment accuracy of PSC to an imageless CAS system in TKA. MethodsThirty-seven patients (41 knees), received a TKA using an imageless CAS system. Subsequently, 38 patients (41 knees), received a TKA using a MRI-based, PSC system.Postoperatively, standing AP hip-to-ankle radiographs were obtained, from which the lower extremity mechanical axis, tibial component varus/valgus, and femoral component varus/valgus mechanical alignment were digitally measured. Each measurement was performed by two blinded, independent observers, and interclass correlations were calculated. A student's two-tailed t test was used to compare the two cohorts (p-value<0.05=significant). ResultsIn the PSC cohort, 70.7% of patients had an overall alignment within 3° of a neutral mechanical axis (vs. 92.7% with CAS, p=0.02), 87.8% had a tibial component alignment within 2° of perpendicular to the tibial mechanical axis (vs. 100% with CAS, p=0.04), and 90.2% had a femoral component alignment within 2° of perpendicular to the femoral mechanical axis (vs. 100% with CAS, p=0.2). Interclass correlation coefficients were good to excellent for all radiographic measurements. ConclusionWhile PSC techniques appear sound in principle, this study did not demonstrate patient specific cutting guides to obtain the same degree of overall mechanical and tibial component alignment accuracy as a CAS technique. Level of evidenceIII: Retrospective cohort study.