INTRODUCTION:. Successful tibial component placement during total knee arthroplasty (TKA) entails accurate rotational alignment, minimal overhang, and good bone coverage, each of which can be facilitated with a tibial component that matches the resected tibial surface. Previous studies investigated bony coverage of multiple tibial component families on digitized resections. However, these studies were based on manual placement of the component that may lead to variability in overhang and rotational alignment. An automated simulation that follows a consistent algorithm for tibial component placement is desirable in order to facilitate direct comparison between tibia component designs. A simulation has been developed and applied to quantify tibial coverage in multiple ethnicities, including Japanese, Indian, and Caucasian. Here, this approach is taken to evaluate tibial coverage of five contemporary tibial designs in Chinese subjects. METHODS:. Digital models of 100 healthy Chinese tibiae (50 male, 50 female; age 68 ± 3 years; stature 1.65 ± 0.10 m) were virtually resected at 5° posterior slope referencing the anterior border of the proximal tibia, 0° varus/valgus rotation referencing the tibial mechanical axis, and 8 mm off the unaffected plateau (reflecting a 10 mm surgical cut, assuming a cartilage thickness of 2 mm). Neutral internal/external (I/E) alignment axis was derived from the medial third of the tubercle and the PCL attachment site. Five commercial tibial designs (Design A, Deluxe™, Montagne, Beijing, China; Designs B-E contemporary market-established symmetric designs from four US manufacturers) were virtually placed on the resected tibiae following an automated algorithm, which maximizes component size while ensuring proper rotational alignment (within 5° I/E) and minimizing overhang (<1 mm in zones 1–4, Fig 1). Tibial coverage (posterior notch excluded, zone 5 in Fig 1) and distance from the component to the exterior cortex of the tibia in four clinically relevant anatomical zones (anterior medial, anterior lateral, posterior medial, and posterior lateral, zones 1–4, Fig 1) were calculated. Statistical significance was defined at p < 0.05. RESULTS:. Coverage across designs varied between 75% and 96%. All five designs showed comparable means and standard deviations in tibial coverage (Fig. 2). Although statistically higher coverage was found in Designs D-E than Designs A-C (p < 0.04), the difference in means (86–87% for Designs A-C; 88% for both Designs D-E) was clinically not meaningful (Fig. 2). Design A was found to be slightly (0.67 mm, p = 0.02) farther away from the cortex than Design E in the anterior medial zone; no other significant differences were found for distance to cortex between any of the component designs in any of the anatomical zones (Fig. 3). DISCUSSION:. The data suggests comparable tibial coverage, which may reflect the likelihood for component subsidence clinically, is expected between the six contemporay design investigtated when implanted into Chinese patients. Though subsidence is multifactorial, and is dependent on aspects of implant design and surgical technique beyond just tibial tray shape, these results nevertheless provide initial indicators on the expected rate of subsidence or overhang in Chinese patients for Design A relative to the more established Designs B-E

Introduction:. Adequate coverage of the resected tibial plateau with the tibial tray is necessary to reduce the theoretical risk of tibial subsidence after primary total knee arthroplasty (TKA). Maximizing tibial coverage is balanced against avoiding excessive overhang of the tray causing soft tissue irritation, and establishing proper tray alignment improving implant longevity and patella function. 1. Implant design factors, including the number of tray sizes, tray shape, and tray asymmetry influence the ability to cover the tibial plateau. 2. Furthermore, rotating platform (RP) tray designs decouple restoring proper tibial rotation from maximizing tibial coverage, which may enhance the ability to maximize coverage. The purpose of the current study was to assess the ability of five modern tray designs (Fig. 1), including symmetric, asymmetric, fixed-bearing, and RP designs, to maximize coverage of the tibial plateau across a large patient population. Methods:. Lower limb computed-tomography scans were collected from 14,791 TKA patients and the tibia was segmented. Virtual surgery was performed with an 8-mm tibial resection (referencing the high side) made perpendicular to the tibial mechanical axis in the frontal plane, with 3° posterior slope, and aligned transversely to the medial third of the tibial tubercle. An automated algorithm placed the largest possible tray on the plateau, optimizing the ML and AP placement (and I-E rotation for the RP tray), to minimize overhang. The largest sized tray that fit the plateau with less than 2-mm of tray overhang was identified for each of the five implant systems. The surface area of the tibial tray was divided by the area of the resected plateau and the percentage of patients with greater than 85% plateau coverage was calculated. Results:. The percentage of patients with greater than 85% plateau coverage across the tray designs ranged from 17.0% to 61.4% (Fig. 1). The tray with the greatest number of size options (Tray 4, 10 sizes) had the best coverage among the fixed-bearing trays. The RP variant of the same tray had the best overall coverage. Tibial asymmetry did not significantly improve the overall tibial coverage across the patient distribution for both asymmetric designs. Incorporating a broader medial condyle improved fit along the posterior medial corner for Tray 2, but increased the average under-hang along the posterior lateral plateau offsetting any improvement in total coverage. Discussion:. This analysis represents the most comprehensive assessment of tray coverage to date across a large TKA-patient population. Large variations exist in the size and shape of the proximal tibia among TKA patients. 3. Developing a tray design which provides robust coverage despite this variation remains challenging. This analysis suggests that tibial asymmetry may not robustly improve coverage. Conversely, incorporating an increased number of tray sizes and utilizing an RP implant to decouple coverage from alignment may provide the most reliable solution for maximizing coverage across the patient population

Introduction:. Tibial component fit, specifically significant overhang of tibial plateau or underhang of cortical bone, can lead to pain, loosening and subsidence. The purpose was to utilize morphometric data to compare size, match, and fit between patient specific and incrementally sized standard unicompartmental knee arthroplasty (UKA) implants. Methods:. CT images of 20 medial UKA knees and 10 lateral UKA knees were retrospectively reviewed. Standard and patient-specific implants were modeled in CAD, utilizing sizing templates and patient-specific CAD Designs. Virtual surgery maximized coverage of tibial plateau while minimizing implant overhang. Tibial plateau implant coverage was evaluated for fit and incidence of overhang/undercoverage. RESULTS:. Patient specific implants provided significantly greater cortical rim coverage versus incrementally sized standard implants, 77% v. 43% (range 41–46%) respectively medially (p < 0.0001) and 60% v. 37% (range 29–41%) laterally (p < 0.0001). Patient-specific and standard implants' arc length were evaluated for percent of implant edge on cortical bone, 84% v. 55% (range 48–59%) medially (p < 0.0001) and 79% v. 57% (range 53–60%) laterally (p < 0.0001). Average amount of overhang/undercoverage of cortical rim area differed in patient-specific and standard implants: 0.24 mm v. 0.46 mm maximum overhang, (p = 0.043); 0.87 mm vs. 3.01 mm maximum undercoverage medially (p < 0.0001); 0.14 mm vs. 0.59 mm maximum overhang, (p = 0.05); 1.19 mm vs. 2.26 mm maximum undercoverage laterally (p = 0.017). Anterior overhang yielded 25 −75% and 30–80% of medial and lateral implants respectively in standard implant group; no overhang in patient-specific implant group. Conclusions:. Tibial plateau anatomy variability produces difficulty optimizing coverage and preventing significant implant overhang/undercoverage with standard unicompartmental implants. Using virtual implantation, standard implants were undersized to avoid overhang. However, we encountered significantly more overhang in standard implants versus patient specific cohort. This study removed variability matching tibial tray and femoral standard group implant placement. Patient-specific implants provide superior cortical bone coverage and fit while minimizing issues of overhang and undercoverage seen in standard implants

Introduction. The goal of tibial tray placement in total knee arthroplasty (TKA) is to maximize tibial surface coverage while maintaining proper rotation. Maximizing tibial surface coverage without component overhang reduces the risk of tibial subsidence. Proper tibial rotation avoids excess risk of patellar maltracking, knee instability, inappropriate tibial loading, and ligament imbalance. Different tibial tray designs offer varying potential in optimizing the relationship between tibial surface coverage and rotation. Patient specific instrumentation (PSI) generates customized guides from an MRI- or CT-based preoperative plan for use in TKA. The purpose of the present study was to utilize MRI information, obtained as part of the PSI planning process, to determine, for anatomic, symmetric, and asymmetric tibial tray designs, (1) which tibial tray design achieves maximum coverage, (2) the impact of maximizing coverage on rotation, and (3) the impact of establishing neutral rotation on coverage. Methods. In this prospective comparative study, MR images for 100 consecutive patients were uploaded into Materialise™ PSI software that was used to evaluate characteristics of tibial component placement. Tibial component rotation and surface coverage was analyzed using the preoperative planning software. Anatomic (Persona™), symmetric (NexGen™), and asymmetric (Natural-Knee II™) designs from a single manufacturer (Zimmer™) were evaluated to assess the relationship of tibial coverage and tibial rotation. Tibial surface coverage, defined as the proportion of tibial surface area covered by a given implant, was measured using Adobe Photoshop™ software (Figure 1). Rotation was calculated with respect to the tibial AP axis, which was defined as the line connecting the medial third of the tibial tuberosity and the PCL insertion. Results. When tibial surface coverage was maximized, the anatomic tray compared to the symmetric/asymmetric trays showed significantly higher surface coverage (82.1% vs 80.4/80.1%; p<0.01), significantly less deviation from the AP axis (0.3° vs 3.0/2.4°; p<0.01), and a significantly higher proportion of cases within 5° of the AP axis (97% vs 73/77%). When constraining rotation to the AP axis, the anatomic tray showed significantly higher surface coverage compared to the symmetric/asymmetric trays (80.8% vs 76.3/75.8%; p<0.01). No significant differences were found between symmetric and asymmetric trays. Discussion. We found that the anatomic tibial tray resulted in significantly higher tibial coverage with significantly less deviation from the AP axis compared to the symmetric and asymmetric trays. When rotation was constrained to the AP axis, the anatomic tray resulted in significantly higher tibial coverage than the symmetric and asymmetric trays. Tibial rotation is recognized as an important factor in the success of a total knee replacement. Maximizing coverage with the least compromise in rotation is the goal for tibial tray design. In this study, the anatomic tibia seemed to optimize the relationship between tibial surface coverage and rotation. This study additionally illustrates the way by which advanced preoperative planning tools (ie. MRI/computer reconstructions) allow us to obtain valuable information with regard to implant design

The goal of tibial tray placement in total knee arthroplasty (TKA) is to maximise tibial surface coverage while maintaining proper rotation. Maximising tibial surface coverage without component overhang reduces the risk of tibial subsidence. Proper tibial rotation avoids excess risk of patellar maltracking, knee instability, inappropriate tibial loading, and ligament imbalance. Different tibial tray designs offer varying potential in optimising the relationship between tibial surface coverage and rotation. Patient specific instrumentation (PSI) generates customised guides from an MRI- or CT-based preoperative plan for use in TKA. The purpose of the present study was to utilise MRI information, obtained as part of the PSI planning process, to determine, for anatomic, symmetric, and asymmetric tibial tray designs, (1) which tibial tray design achieves maximum coverage, (2) the impact of maximising coverage on rotation, and (3) the impact of establishing neutral rotation on coverage. MR images for 100 consecutive patients were uploaded into Materialise™ PSI software that was used to evaluate characteristics of tibial component placement. Tibial component rotation and surface coverage was analysed using the preoperative planning software. Anatomic (Persona™), symmetric (NexGen™), and asymmetric (Natural-Knee II™) designs from a single manufacturer (Zimmer™) were evaluated to assess the relationship of tibial coverage and tibial rotation. Tibial surface coverage, defined as the proportion of tibial surface area covered by a given implant, was measured using Adobe Photoshop™ software. Rotation was calculated with respect to the tibial AP axis, which was defined as the line connecting the medial third of the tibial tuberosity and the PCL insertion. When tibial surface coverage was maximised, the anatomic tray compared to the symmetric/asymmetric trays showed significantly higher surface coverage (82.1% vs 80.4/80.1%; p<0.01), significantly less deviation from the AP axis (0.3° vs 3.0/2.4°; p<0.01), and a significantly higher proportion of cases within 5° of the AP axis (97% vs 73/77%). When constraining rotation to the AP axis, the anatomic tray showed significantly higher surface coverage compared to the symmetric/asymmetric trays (80.8% vs 76.3/75.8%; p<0.01). No significant differences were found between symmetric and asymmetric trays. We found that the anatomic tibial tray resulted in significantly higher tibial coverage with significantly less deviation from the AP axis compared to the symmetric and asymmetric trays. When rotation was constrained to the AP axis, the anatomic tray resulted in significantly higher tibial coverage than the symmetric and asymmetric trays. Tibial rotation is recognised as an important factor in the success of a total knee replacement. Maximising coverage with the least compromise in rotation is the goal for tibial tray design. In this study, the anatomic tibia seemed to optimise the relationship between tibial surface coverage and rotation. This study additionally illustrates the way by which advanced preoperative planning tools (ie. MRI/computer reconstructions) allow us to obtain valuable information with regard to implant design

INTRODUCTION. Balancing accurate rotational alignment, minimal overhang, and good coverage during total knee arthroplasty (TKA) often leads to compromises in tibial component fit, especially in smaller-sized Asian knees. This study compared the fit and surgical compromise between contemporary anatomic and non-anatomic tibial designs in Japanese patients. METHODS. Size and shape of six contemporary tibial component designs (A:anatomic, B:asymmetric, C-F:symmetric) were compared against morphological characteristics measured from 120 Japanese tibiae resected following TKA surgical technique. The designs were then digitally placed on the resected tibiae. Each placement selected the largest possible component size, while ensuring <1mm overhang and proper alignment (within 5° of neutral rotational axis). When a compromise on either alignment or overhang was required (due to smaller-sized component unavailable), the design was flagged as “no suitable component fit” for that bone. Tibial coverage was compared across designs. Next, 32 femora were randomly selected from the dataset onto which each design was evaluated in two placements, the first maximizing coverage without attention to rotation and the second enforcing rotational accuracy. Downsizing was identified if in the second placement, enforcing rotational accuracy, required a smaller component size compared the first placement. The degree of mal-alignment while maximizing coverage, the incidence of downsizing, and difference in coverage between the two placements were compared across designs. Statistical significance was defined at p<0.05. RESULTS. Design A closely matched the tibial morphology and had better size and shape conformity than Designs B-F (select metrics shown in Fig. 1). Design A exhibited higher average coverage (92%) than other designs in all ethnicities (85–87%, Fig. 2A) (p<0.01). Designs D-F had no suitable component fit in 1.6–2.4% of the bones (Fig. 2B). Coverage generally decreased with reduced component size (Fig.2C), with Design A having higher coverage than Designs B-F across all sizes. In the randomly selected 32 tibiae, enforcing rotational accuracy significantly compromises coverage in Designs B-F (Fig.3A) (p<0.01), with up to 15% in individual bones. In contrast, coverage of Design A was not influenced by enforcing rotational accuracy (p=0.52). Designs B-F were found to require downsizing on 41–66% of bones due to >5° rotation, with components internally rotated beyond 10° on 31–59% of the bones (Fig.3B). In contrast, Design A required downsizing on only 6% of the bones, caused by small mal-rotations (<10°). Designs B-D and F required downsizing of ≥2 sizes on 3–16% of bones; while a single downsize was sufficient for Design A (Fig.3C). DISCUSSION. The anatomic design not only has the closest match to the natural tibia, but also consistently has the highest coverage across bone sizes. It also exhibits fewer incidences of downsizing and reduced propensity for mal-alignment than the non-anatomic designs investigated. In contrast, in the non-anatomic tibial component designs, ensuring rotation accuracy considerably compromised tibial coverage. This result, suggests that many non-anatomic designs do not fully accommodate variations in bone anatomy in the Japanese patients, thus forcing a compromise

Introduction. Malpositioning of the tibial component is a common error in TKR. In theory, placement of the tibial tray could be improved by optimization of its design to more closely match anatomic features of the proximal tibia with the motion axis of the knee joint. However, the inherent variability of tibial anatomy and the size increments required for a non-custom implant system may lead to minimal benefit, despite the increased cost and size of inventory. This study was undertaken to test the hypotheses: . 1. That correct placement of the tibial component is influenced by the design of the implant. 2. The operative experience of the surgeon influences the likelihood of correct placement of contemporary designs of tibial trays. Materials and Methods. CAD models were generated of all sizes of 7 widely used designs of tibial trays, including symmetric (4) and asymmetric (3) designs. Solid models of 10 tibias were selected from a large anatomic collection and verified to ensure that they encompassed the anatomic range of shapes and sizes of Caucasian tibias. Each computer model was resected perpendicular to the canal axis with a posterior slope of 5 degrees at a depth of 5 mm distal to the medial plateau. Fifteen joint surgeons and fourteen experienced trainees individually determined the ideal size and placement of each tray on each resected tibia, corresponding to a total of 2030 implantations. For each implantation we calculated: (i) the rotational alignment of the tray; (ii) its coverage of the resected bony surface, and (iii) the extent of any overhang of the tray beyond the cortical boundary. Differences in the parameters defining the implantations of the surgeons and trainees were evaluated statistically. Results. On average, the tibial tray was placed in 5.5 ± 3.1° of external rotation. The overall incidence of internal rotation was only 4.8%: 10.5% of trainee cases vs. 0.7% of surgeon cases (p < 0.0001). The incidence of internal rotation varied significantly with implant design, ranging from 1.7% to 6.2%. Bony coverage averaged 76.0 ± 4.5%, and was less than 70% in 8.6% of cases. Tibial coverage also varied significantly between designs (73.2 ± 4.3% to 79.2 ± 3.8%; p < .0001). Clinically significant cortical overhang (>1 mm), primarily in the posterior-lateral region, was present in 12.1% of cases, and varied by design, as expressed by the area of the tray overhanging the cortical boundary (min: 2.3 ± 6.7 mm. 2. ; max: 4.7 ± 7.9 mm. 2. ; p < .0001). The surgeons and the trainees also differed in terms of the incidence of sub-optimal tibial coverage (10.0% vs. 14.4%, p < 0.001), and cortical overhang (7.4% vs. 9.7%, p < 0.001). Discussion. 1. Malrotation, bony coverage and cortical overhang are all strongly influenced by the design of the tibial tray selected and the experience of the surgeon. 2. Compared to trainees, experienced surgeons tend to position tibial trays in more external rotation, and with less concern for reduced bony coverage and cortical overhang than trainees. 3. This study supports the hypothesis that improvements in the outcome and reliability of TKR may be achieved through attention to implant design

Enhanced appreciation of normal knee kinematics and the inability to replicate these in the replaced total knee has led to increased enthusiasm for partial knee arthroplasty by some. These arthroplasties more closely replicate normal kinematics since they inherently preserve the anterior cruciate ligament (ACL). Indications for medial UKA are: anteromedial osteoarthritis with an intact ACL, posterior cruciate ligament, and medial collateral ligament (MCL), full thickness cartilage loss, and correctable deformity demonstrated radiographically with valgus stress view; full thickness cartilage laterally with no central ulcer; <15 degrees of flexion contracture, < 15 degrees varus and > 90 degrees flexion. The state of the patellofemoral joint, chondrocalcinosis, obesity, age and activity level are NOT contraindications to medial mobile-bearing UKA. The only certain contraindications are the presence of inflammatory arthritis or a history of previous high tibial osteotomy (HTO). Advantages of medial UKA are that it preserves undamaged structures, it is a minimally invasive technique with low incidence of perioperative morbidity, preservation of the cruciate mechanism results in more “normal” kinematics versus TKA, it normalises contact forces and pressures in the patellofemoral joint, and it provides better range of motion than TKA. Furthermore, medial UKA results in better function than TKA in gait studies, with demanding activities, such as climbing stairs, having a better “feel”. Pain relief with medial UKA is equivalent or better than TKA, and morbidity and mortality are decreased compared with TKA, as well as venous thromboembolism. Recommended preoperative imaging studies consist of plain radiographs with the following views obtained: standing AP, PA flexed, lateral, Merchant or axial, and valgus stress. There are several surgical perils associated with performing medial UKA. First, in regard to patient selection, avoid medial UKA in patients with residual hyaline cartilage – the joint must be bone on bone. Second, perform a conservative tibial resection with respect to depth to prevent tibial collapse as well as excessive overload of weakened bone, and avoid excessive posterior slope. Perform the tibial resection coplanar with tibial spine/ACL insertion to maximise tibial coverage. Avoid overcorrection of deformity. Do not perform a medial release. Balance flexion/extension gaps meticulously. For mobile-bearing designs, remove all impinging osteophytes. Over 55 published studies report results with mobile-bearing medial UKA, with survival ranging 63.2–100% at mean follow-up ranging from 1 to 17.2 years

AIM. Tibial component design has be been scrutinized in a number of studies in an attempt to improve tibial coverage in total knee arthroplasty. However, very few have controlled for both component rotation and resultant changes to posterolateral tibial tray overhang and posteromedial underhang. We hypothesize that asymmetrical tibial components can provide greater coverage than symmetrical trays without increasing overhang. METHODS. The 6 most commonly used tibial trays on the Australian Joint Registry (2009) were superimposed on MRI slices of normal knees to assess tibial component overhang, underhang and percent coverage. Rotational alignment in this analysis was based upon the line joining the junciton of the medial and middle 1/3 of the patellar tendon and the PCL insertion. RESULTS. The popliteus tendon was on average 1 mm from the posterior tibial cortex. Only 28.2% of all tibial trays showed optimal posterolateral fit and 48.8% were oversized enough to cause popliteus impingement. NexGen symmetric tray had the largest number of optimally fitting trays on the posterolateral corner (33.7%, the difference was significant against the Genesis II and Triathlon only). The asymmetric Genesis II had the largest percentage of overhang greater than 1 mm. All 6 tray designs had over 80% tibial bone coverage. The Genesis II had the greatest amount of coverage at 88% (paired t test, p<0.001 for each comparison). CONCLUSION. Asymmetric trays in the analysis appear to offer improved bone coverage at the expense of tray overhang when compared to symmetric tray designs thus rejecting our hypothesis

Introduction:. Appropriate transverse rotation of the tibial component is critical to achieving a balance of tibial coverage and proper tibio-femoral kinematics in total knee replacement (TKR), yet no consensus exists on the best anatomic references to determine rotation. Historically, surgeons have aligned the tibial component to the medial third of the tibial tubercle. 1. , but recent literature suggests this may externally rotate the tibial component relative to the femoral epicondylar axis (ECA) and that the medial border of the tubercle is more reliable. 2. Meanwhile, some TKR components are designed with asymmetry of the tibial tray assuming that maximizing component coverage of the resected tibia will result in proper alignment. The purpose of this study was to determine how different rotational landmarks and natural variation in osteoarthritic patient anatomy may affect asymmetry of the resected tibial plateau. Methods:. Pre-operative computed-tomography scans were collected from 14,791 TKR patients. The tibia and femur were segmented and anatomic landmarks identified: tibial mechanical axis, medial third and medial border of the tibial tubercle, PCL attachment site, and the surgical ECA of the femur. Virtual surgery was performed with an 8-mm resection (referencing the high side) made perpendicular to the tibial mechanical axis in the frontal plane, with 3° posterior slope, and transversely aligned with three different landmarks: the ECA, the medial border, and medial third of the tubercle. In each of these rotational alignments, the relative asymmetry of the medial and lateral plateaus was calculated (Medial AP/Lateral AP) (Fig. 1). Results:. Rotational alignment of the tibial component to the ECA, medial border, and medial third of the tubercle resulted in progressive external rotation of the tibial tray on the bone. Alignment to the medial border and medial third of the tubercle resulted in average 0.9° ± 5.7° and 7.8° ± 5.3° external rotations of the tray relative to the ECA, respectively (Fig. 2). Greater external rotation of the tibial implant relative to the bone increased the appearance of tibial asymmetry (Fig. 3). Referencing the medial border and medial third of the tubercle resulted in apparent tibial bone asymmetry of 1.10 ± 0.10 and 1.12 ± 0.10, respectively. Discussion:. Assuming the ECA is the appropriate rotational reference to re-establish appropriate kinematics. 2. , alignment to the medial border of the tubercle resulted in the most favorable tray alignment. However, there was a great deal of variation between the relative position of the ECA and the tubercle across the patient population. Rotational alignment to either the medial border or medial third of the tubercle resulted in external tray alignment relative to the ECA of greater than 3 degrees for 36% and 84% of patients, respectively. In addition, increased tray asymmetry (broader medial plateau) necessitates relative external rotation of the tray on the bone reducing the flexibility of intra-operative rotational adjustment. Tray asymmetry greater than 1.10 (the asymmetry of the resected tibia when aligned to the ECA) may result in external mal-rotation for a significant portion of the patient population

INTRODUCTION:. Proper tibial rotation has been cited as an important prerequisite to optimal total knee replacement. The most commonly recognized rotational landmark is the medial 1/3. rd. of the tibial tubercle. The purpose of this study was to quantify the amount of variability this structure has from a common reference as well as to understand the effects of component design when referencing this structure. METHODS:. Subjects were prospectively scanned into a Virtual Bone Database (Stryker Orthopaedics, Mahwah, NJ), which is a collection of body CT scans from subjects collected globally. All CT scans displayed cropped bones were excluded. SOMA™ (Stryker) is a unique tool with the ability to take automated measurements of quantities such as distances and angles on a large number of pre-segmented bone samples which was then to perform calculations represented in this study. Demographic information for each subject was recorded were known. For the analysis, the mechanical axis of the tibia (MAT) was established by connecting the center of the proximal tibia to the center of the ankle. From the MAT, a perpendicular resection plane was made at a distance of 9 mm from the most proximal portion of the lateral condyle. This plane was then used as a virtual resection plane to establish the points for the remaining structures which was the medial 1/3. rd. of the tibial tubercle and the posterior notch of the PCL insertion. The following axes were identified: 3TT (line between the medial 1/3. rd. of the tibial tubercle and the posterior notch of the tibia); 3CTT (line between the medial 1/3. rd. of the tibial tubercle and the center of the tibia); and the posterior axis of the tibia (line connecting the two most posterior points of the tibia at the virtual resection plane). Measurements made were the angle of the 3TT Line to the posterior axis and the angle of the 3CTT Line to the posterior axis. RESULTS:. CT Scans of the Left Knees (n = 524), Right Knees (n = 527), and combined left/right knee (n = 1051) were collected for this study. The mean 3TT angle for the left knee was 74.6° ± 3.0° (Range: 60.2°–84.8°) and right knee was 74.5° ± 3.0° (Range: 65.1°– 85.1°). The combined (left/right) angle was 74.5° ± 3.0° (Range: 60.2°–85.1°). The mean 3CTT angle for the left knee was 71.2° ± 3.6° (Range: 57.6°–83.2°) and right knee was 71.1° ± 3.5° (Range: 61.4°–82.3°). The combined (left/right) angle was 71.1° ± 3.6° (Range: 57.6°–83.2°). The two methods resulted in a 3.4° difference, with the 3TT reference being more externally rotated. DISCUSSION:. The tibial tubercle is a common landmark used to set the rotation of the tibial component and utilizing the posterior aspect of the tibia provides a common reference point to establish variations that could exist with this landmark. The amount of variation of the tibial tubercle can vary by over 25 degrees. Asymmetric baseplates will set rotation based on tibial coverage so variation from the tubercle is can not be accommodated if the surgeon routinely uses this as a landmark. Symmetric baseplates can provide more options for rotational placement

Introduction:. Malrotation of the tibial component is a common error in TKR, and has been frequently cited as the cause of clinical symptoms. Correct rotational orientation of the tibial tray is difficult to achieve because the resected surface of the tibia is internally rotated and is not symmetrical in shape. This suggests that anatomically contoured components may lead to improved rotational positioning. This study was undertaken to test the hypotheses: . 1. Use of an anatomically shaped tibial tray can reduce the prevalence of malrotation and cortical over-hang in TKA while increasing coverage of the resected tibial surface, and. 2. Component shape has more influence on the results of surgical trainees compared to experienced surgeons. Materials and Methods:. A standard symmetric design of tibial tray was developed from the profiles of 3 widely used contemporary trays. Corresponding asymmetric profiles were generated to match the average shape of the resected surface of the tibia based on a detailed morphometric analysis of anatomic data. Both designs were proportionally scaled to generate a set of 7 different sizes. Computer models of eight tibias were selected from a large anatomic collection. The proximal tibia was resected perpendicular to the canal axis with a posterior slope of 5 degrees at a depth of 5 mm (medial). Eleven experienced joint surgeons and twelve trainees individually determined the ideal size and placement of each tray on each of the 8 resected tibias. The rotational alignment, coverage of the resected bony surface, and extent of overhang of the tray beyond the cortical boundary were measured for each implantation. Differences in the parameters defining the implantations of the surgeons and trainees were evaluated statistically. Results:. Bony coverage was significantly greater with the asymmetric vs. the symmetric design (87.0 ± 4.1% vs. 75.6 ± 4.0%; p < 0.0001). Coverage was less than 75% in 37% of symmetric trays, whereas the worst coverage obtained with the asymmetric design was 77.0%. Clinically significant cortical overhang (>1 mm) was present in 35% of symmetric vs. 11% of asymmetric cases (p < 0.0001). On average, the asymmetric tray was placed in 4.1 ± 3.7° of external rotation vs. 1.6 ± 4.6° for the symmetric tray (p < 0.0001). The tray was implanted in some degree of internal rotation in 24% of cases, 15% for the asymmetric design vs. 33% for the symmetric (p < 0.0001). There was minimal difference between the results of implantations performed by trainees vs. experienced surgeons, in terms of tibial coverage (p = 0.245), cortical overhang (p = 0.735), or the prevalence of internal rotation (p = 0.147). Trainees placed 6.3% of all cases in severe internal rotation (>5°) compared with 12.5% of surgeon cases (p = 0.154). Discussion. 1. The incidence of malrotation was substantially less with anatomical vs symmetrical tibial trays. 2. The asymmetric design was also associated with a large reduction in cortical overhang and increased coverage of the resected tibial surface. 3. There was no overall difference between the performance of trainees and experienced joint surgeons, regardless of the design of the implant. This suggests that current training and surgical guides are inadequate in achieving correct positioning of the tibial component in TKR