Mobile medial pivot (lateral slide)-type total knee arthroplasty exhibited different motion patterns between under anaesthesia and weight-bearing condition.

PURPOSE: Total knee arthroplasty (TKA), which has medial pivot and mobile-bearing mechanisms, has been developed and clinically used. However, the in vivo dynamic kinematics of the mobile medial pivot-type TKA (MMPTKA) is unclear. This study analysed the in vivo kinematics of MMPTKA in weight-bearing and nonweight-bearing conditions. METHODS: The study included 10 knees that underwent primary TKA using MMPTKA. After TKA, lateral view radiographs of the knee in full extension, 90° of flexion and passive full flexion were taken under general anaesthesia in the nonweight-bearing condition. At least 6 months postoperatively, knee motion during squatting from a weight-bearing standing position was observed using a flat-panel detector and analysed using the three-dimensional-to-two-dimensional image registration technique. RESULTS: Under anaesthesia: in passive full flexion, the anteroposterior (AP) locations of the femoral component's medial and lateral distal points were 10.2 and 16.0 mm posterior, and the rotational angles of the femoral component's X-axis (FCX) and insert were 8.1° external rotation and 18.5° internal rotation to full extension, respectively. Squatting: the AP translations of the femoral component's medial and lateral most distal points were 2.2 and 6.4 mm, and the rotational angles of the FCX and insert were 5.7° and 1.6° external rotation, respectively. Significant differences were observed in the AP translation of the femoral component's medial and lateral most distal points and changes in the insert's rotational angle when comparing under anaesthesia and squatting. CONCLUSIONS: The kinematics of the insert in MMPTKA was significantly influenced by loading and muscle contraction. The femoral component exhibited substantial external rotation and posterior translation under anaesthesia, which may contribute to achieving an optimal range of motion. The insert remained relatively stable during squatting and minimal rotation was observed, indicating good stability. MMPTKA was expected to demonstrate rational kinematics by incorporating mobile and medial pivot mechanisms. LEVEL OF EVIDENCE: Level IV, prospective biomechanical case series study.

Variability of functional knee phenotype for coronal alignment in advanced varus knee osteoarthritis in the Japanese population.

PURPOSE: New concept of functional knee phenotypes in Caucasians demonstrated the variability of coronal alignment in knee osteoarthritis (OA), but it remains unclear in Japanese. This study aims to analyze the knee phenotype in advanced varus knee OA for Japanese. In addition, the ethnical difference is discussed. METHODS: This study analyzed 879 knees involving 186 males (74 years) and 693 females (74 years). The knee phenotypes were assessed by the definition in Hirschmann's group. The hip-knee-ankle angle (HKA), femoral mechanical angle (FMA) and tibial mechanical angle (TMA) were assessed in CT data according to the coordinate system. The neutral angle was 180° in HKA, 93° in FMA and 87° in TMA. The smaller angle means larger varus angles. RESULTS: The average angle (males, females) of the HKA (170.9 ± 4.3°, 169.4 ± 5.0°), FMA (91.5 ± 2.7°, 90.6 ± 3.0°), and TMA (82.4 ± 3.6°, 82.7 ± 3.7°) demonstrated varus angles with the sex difference (HKA, p < 0.001; FMA, p = 0.001). The phenotypes were 73 types in males and 150 types in females with a mild correlation between the HKA and the FMA or TMA. In 61.3% of males and 52.2% of females, the TMA was greater than the FMA, while the FMA was greater in 16.7% of males and 23.1% of females. CONCLUSION: There were many functional knee phenotypes with sex differences for advanced varus knee OA in Japanese, showing ethnical differences of larger varus angles compared to those for Caucasians in the previous report. LEVEL OF EVIDENCE: IV.

The Matsudai Knee Osteoarthritis Survey showed the longitudinal changes of knee phenotypes in alignment and structure during 23-28 years.

PURPOSE: The longitudinal changes in alignment and structure, including the joint line and cortical bone thickness (CBT) of the femur and tibia, and knee phenotype in patients with knee osteoarthritis (OA) remain unknown. The aim of this retrospective study was to clarify the longitudinal changes in matched healthy subjects. METHODS: The follow-up Matsudai Knee Osteoarthritis Survey was administered between 23 and 28 years. This study included 285 healthy knees from 235 females with an average age of 53 ± 6 years at baseline. The non-OA individuals, with an average age of 79 ± 4 years, were divided into three groups at baseline according to their follow-up radiographic results [the non-OA (n = 52), early OA (n = 131), and advanced OA groups (n = 102)]. Changes in alignment, joint line, CBT, and knee phenotype were assessed at baseline and at follow-up using standing anteroposterior radiographs. RESULTS: This study showed significant varus changes in the alignment (p < 0.001) and tibial and femoral joint line parameters (p < 0.05) in the OA group. Decreased CBT and increased mediolateral CBT ratios were observed in all groups (p < 0.001). The knee phenotypes in the OA groups were changed to varus angles, especially in the alignment and tibial joint line. CONCLUSIONS: The longitudinal changes of knee phenotypes in alignment and structure (CBT and joint line) from baseline to follow-up were shown in the OA groups. In addition, alignment and tibial structural factors at baseline are useful in predicting the incidence of knee OA in daily practice. LEVELS OF EVIDENCE: III.

The medial inclination of the proximal tibia is associated with the external knee adduction moment in advanced varus knee osteoarthritis.

PURPOSE: Whether the inclined articular surface on the medial proximal tibia and the external knee adduction moment (KAM) correlate remains unclear. The hypothesis was that a steeper inclined articular surface correlated with a larger KAM in advanced knee osteoarthritis (OA). METHODS: A total of 44 females (non-OA, 9 knees; early OA, 14 knees; advanced OA, 21 knees; mean age, 58 ± 16 years) were examined. Three-dimensional (3D) assessment was used on biplanar long-leg radiographs and 3D bone models using a 3D to 2D image registration technique. The approximation plane in the proximal tibia was determined using the least-square method. The joint moments were mathematically calculated in a gait analysis, applying a motion capture system and force plates. The main evaluation parameters were the femorotibial angle (FTA), the coronal inclination of the approximation plane in the medial proximal tibia (coronal inclination), and internal knee joint moments. The KAM means the external moments balanced with the internal knee abduction moments. RESULTS: The advanced OA showed a larger internal abduction moment (p = 0.017) at the loading response than the other groups. The larger FTA and steeper coronal inclination correlated with the larger internal abduction moment (FTA, p < 0.001; coronal inclination, p = 0.003) at the loading response. CONCLUSIONS: As the clinical relevance, the association among the coronal inclination of the medial proximal tibia, lower extremity alignment, and KAM is one of the key factors to help better understand the etiology of knee OA. LEVEL OF EVIDENCE: III.

The coronal inclination of the medial tibial plateau affects coronal gait kinematics for varus osteoarthritic knees.

PURPOSE: This study aims to (1) measure the kinematics of lower extremity alignment and the bony position relative to the ground during walking, focusing on the coronal plane, and (2) determine the correlation between the kinematics and coronal inclination of the medial tibial plateau (coronal inclination) for healthy and varus knee osteoarthritis (OA). METHODS: In this study, 43 women (non-OA, 9 knees; early OA, 13 knees; advanced OA, 21 knees; mean age 58 ± 17 years) were examined. The knee phenotypes in varus knee OA were varied. Three-dimensional (3D) knee kinematics were calculated in gait analysis by combining the motion capture system and the 3D lower extremity alignment assessment system via biplanar long-leg X-rays, applying the 3D-2D registration technique. The main parameters were the kinematics of the bony axes relative to the ground in the coronal plane during the stance phase of the gait. The differences in overall kinematics were assessed using repeated measures ANOVA with Tukey's post hoc test. The association between kinematic parameters and coronal inclination was evaluated by multiple linear regression after univariate analysis. RESULTS: The tibia tilted laterally during the loading response, and a plateau area subsequently appeared until the terminal stance phase, whereas the femur slowly tilted laterally until the terminal stance phase. The dynamic alignment showed a relatively large varus angular change during the loading response in all groups. The trend of motion was similar among all groups (p = n.s.), although to varying degrees. The coronal inclination was the more dominant factor than the Kellgren-Lawrence (K-L) grades (ß = - 0.423, p = 0.005) when the change in dynamic alignment was determined. CONCLUSIONS: The TAA plateau area after the loading response implies that the tibial articular surface may become horizontal. The femur slowly tilted laterally until the terminal stance phase in response to the tibial motion. Consequently, the dynamic alignment showed a varus angular change, in which coronal MCT was more involved than K-L grades.

Predictive factors for developing osteochondritis dissecans after surgery for discoid lateral meniscus are younger age and shorter meniscal width.

PURPOSE: This study aimed to identify the predictive factors for postoperative osteochondritis dissecans (OCD) in juvenile and adolescent knees with discoid lateral meniscus (DLM). METHODS: In total, 242 patients with symptomatic DLM who underwent surgery were identified. Inclusion criteria were set as follows: (1) age ≤ 17 years with an open growth plate, (2) follow-up magnetic resonance imaging, and (3) absence of preoperative OCD. Consequently, 52 patients were retrospectively investigated. Average age during surgery, body mass index (BMI), and follow-up duration were 12 years [95% confidence interval (CI) 11-13], 19.2 kg/m2 (95% CI 18.4-20.1), and 27.3 months (95% CI 20.9-33.7), respectively. Age, sex, sports activities, BMI, symptomatic OCD in other joints, postoperative rehabilitation, preoperative shift of DLM by Ahn's classification, surgical procedures (saucerization alone or with stabilization, and subtotal meniscectomy), and postoperative meniscal width were analyzed as possible predictive factors. RESULTS: Postoperatively, 42 patients without OCD and 10 with OCD were observed. In univariate analysis, younger age [odds ratio (OR) 1.5; p = 0.003], subtotal meniscectomy (OR 6.3; p = 0.027), and shorter meniscal width (OR 2.7; p = 0.005) were predictive factors for postoperative OCD. Multivariate analysis demonstrated that younger age (OR 1.6; p = 0.009) and shorter meniscal width (OR 1.5; p = 0.003) were predictive factors. CONCLUSIONS: To prevent postoperative OCD after DLM surgeries, achieving stabilization with adequate meniscal width is necessary for juvenile knees. LEVEL OF EVIDENCE: III.

The postoperative shorter meniscal width was the risk factor of lateral meniscal extrusion in the middle portion for juvenile and adolescent knees with discoid lateral meniscus.

PURPOSE: The study aim was to clarify the risk factors for postoperative meniscal extrusion in a middle portion in juvenile and adolescent knees with DLM. METHODS: Forty-six patients with symptomatic DLM who underwent surgery were retrospectively assessed. Inclusion criteria were set as follows: (1) aged ≤ 17 years with an open growth plate, (2) preoperative and postoperative follow-up MRI, and 3) reshaping surgeries comprising of saucerization alone or with meniscal repair. Average (95%CI) age during surgery, body mass index (BMI), and follow-up duration were 12 years (11-13), 19.9 kg/m2 (18.7-21.0), and 26.4 months (19.5-33.3), respectively. Age, sex, sports activities, BMI, postoperative rehabilitation, preoperative shift of DLM by Ahn's classification, surgical procedures, postoperative meniscal width of all portions, and meniscal healing were analyzed. RESULTS: Postoperatively, eight knees in the no-extrusion group and 38 knees in the extrusion group were observed. In the univariate logistic regression analysis, shorter meniscal width in a middle portion (OR = 1.580, p = 0.006), shorter minimum width of all portions (OR = 1.674, p = 0.024), and meniscal healing (OR = 0.160, p = 0.028) were the risk factors for meniscal extrusion in a middle portion. Multiple logistic regression analysis demonstrated that shorter meniscal width in a middle portion was the risk factor. CONCLUSIONS: As the clinical relevance, to prevent postoperative meniscal extrusion of the middle portion with DLM, surgeons are necessary to pay attention to maintain the adequate meniscal width for juvenile and adolescent knees. LEVEL OF EVIDENCE: III.

Age- and sex-related characteristics in cortical thickness of femoral diaphysis for young and elderly subjects.

INTRODUCTION: Cortical thickness of the femoral diaphysis is assumed to be a preferred parameter in the assessment of the structural adaptation by mechanical use and biological factors. This study aimed to investigate the age- and sex-specific characteristics in cortical thickness of the femoral diaphysis between young and elderly non-obese people. MATERIALS AND METHODS: This study investigated 34 young subjects (21 men and 13 women; mean age: 27 ± 8 years) and 52 elderly subjects (29 men and 23 women; mean age: 70 ± 6 years). Three-dimensional (3D) cortical thickness of the femoral diaphysis was automatically calculated for 5000-8000 measurement points using the high-resolution cortical thickness measurement from clinical CT data. In 12 assessment regions created by combining three heights (proximal, central, and distal diaphysis) and four areas of the axial plane at 90° (medial, anterior, lateral, and posterior areas) in the femoral coordinate system, the standardized thickness was assessed using the femoral length. RESULTS: As per the trends, (1) there were no differences in medial and lateral thicknesses, while the posterior thickness was greater than the anterior thickness, (2) the thickness in men was higher than that in women, and (3) the thickness in young subjects was higher than that in elderly subjects. CONCLUSIONS: The results of this study are of clinical relevance as reference points to clarify the causes of various pathological conditions for diseases of the lower extremities.

The inclination of the femoral medial posterior condyle was almost vertical and that of the lateral was tilted medially.

PURPOSE: The purpose of this study was to three-dimensionally analyse the size and shape of the femoral posterior condyles of the normal knee. METHODS: A total of 62 healthy Japanese volunteers (37 males and 25 females) providing a sample of 124 normal knee joints, who had no knee-related symptoms and no history of major trauma, underwent computed tomography scans of the bilateral femur and tibia. Three-dimensional digital models of the femur were constructed from computed tomography data using visualisation and modelling software. The following parameters were evaluated: (1) the radii of the posterior condyles approximated to spheres and (2) the inclination angle of the posterior condyles in the coronal plane of the femoral coordinate system. RESULTS: The radii of the medial and lateral condyles approximated to spheres were 17.0 ± 1.6 and 17.1 ± 1.8 mm, respectively and were not different. The inclination angles of the medial and lateral condyles in the coronal plane were - 0.6° ± 4.6° and 9.7° ± 5.7°, respectively. The medial condyle was almost vertical, whereas the lateral one was medially tilted. CONCLUSIONS: This study found an asymmetrical inclination between medial and lateral condyles. This may be related to the asymmetrical motion of the knee, which is known as medial pivot motion. This finding provides valuable morphological information and may be useful for implant designs for total knee arthroplasty. LEVEL OF EVIDENCE: IV.

The majority of patellar avulsion fractures in first-time acute patellar dislocations included the inferomedial patellar border that was different from the medial patellofemoral ligament attachment.

PURPOSE: Accurate assessment of the locations of patellar avulsion fractures in acute patellar dislocations is clinically relevant for decision making for treatment. The study aim was to classify the locations of patellar avulsion fractures with a focus on the ligament attachments of medial stabilizing structures. METHODS: Out of 131 first-time acute traumatic patellar dislocations, 61 patients had patellar fractures. Subsequently, 10 patients with isolated osteochondral fractures of the articular surface in the patella were excluded. Finally, 51 patients (34 females and 17 males, average age: 18.5 years, 95% CI 16.1-20.9) were included in the study cohort. Based on the locations of the patellar attachment, the patients were divided into three groups: the superior group [medial patellofemoral ligament (MPFL) attachment], inferior group [medial patellotibial ligament (MPTL)/medial patellomeniscal ligament (MPML) attachment], and mixed group. RESULTS: In the patellar avulsion group (51 patients), the superior group, mixed group, and inferior group contained 8/51 (16%), 12/51 (24%), and 31/51 (61%) patients, respectively. CONCLUSIONS: This study showed that 84% of the patellar avulsion fractures were located in the inferomedial patellar border, which consisted of MPTL/MPML attachments that were clearly different from the true "MPFL" attachment at the superomedial patellar border. In terms of the clinical relevance, the acute surgical repair of MPTL/MPML attachments in the inferomedial patellar border may not sufficiently control the patella if optimal management of the MPFL is not performed. LEVEL OF EVIDENCE: IV.

Articular surface of the medial proximal tibia is aligned parallel to the ground in three-dimensional space under weight-bearing conditions in healthy and varus osteoarthritic knees.

PURPOSE: To test the hypothesis that an inclined articular surface on the medial proximal tibia is aligned more parallel to the ground in three-dimensional (3D) space under weight-bearing (WB) conditions (parallel phenomenon) than under non-WB (NWB) conditions in healthy and varus osteoarthritic knees. METHODS: We examined 55 healthy knees (26 women, 29 men; mean age, 70 ± 6 years) and 108 varus osteoarthritic knees (66 women, 16 men; mean age, 74 ± 7 years). For the evaluation under WB conditions, a 3D assessment system was used on biplanar long-leg radiographs and 3D bone models using a 3D-to-2D image registration technique. In addition, the least square method was used to determine the approximation plane. The angles between the normal vector for the approximation plane of an articular surface on the medial proximal tibia and each axis of the tibial or world coordinate system were calculated. RESULTS: Morphologically, the inclination of the approximation plane was steeper in osteoarthritic knees than in healthy knees (p < 0.0001). The approximation plane was aligned more parallel to the ground under WB conditions than under NWB conditions in healthy (p < 0.0001) and osteoarthritic knees (p < 0.0001). CONCLUSIONS: The parallel phenomenon in the medial proximal tibia was confirmed for healthy and varus osteoarthritic knees. The medial proximal tibia plays an important role in the parallel phenomenon, assumingly associated with varus alignment and varus thrust. The inclination of the medial proximal tibia may become a new parameter for imaging investigations. LEVEL OF EVIDENCE: III.

Scoring system for optimal management of acute traumatic patellar dislocation: A multicenter study.

BACKGROUND: In first-time acute traumatic patellar dislocations, numerous anatomic and clinical factors are complicatedly associated and should be comprehensively considered for the optimal management of conservative or surgical intervention. The purpose of this study was to establish a scoring system, using a concept that is not complicated and easily used in clinic. METHODS: Of 131 first-time acute traumatic patellar dislocations, 81 patients (51 females, 30 males) with an average age of 19.6 (95%CI, 17.5-21.6) years and an average follow-up duration of 17.1 (95% CI, 12.3-21.9) months, who underwent conservative treatment after first-time dislocations, were reviewed. Based on the odds ratios applying logistic regression analysis, the scoring system was established. RESULTS: The scoring system (total: 10 points) had simple composition of age <20 (2 points), sports injury (1 points), hemarthrosis (1 points), and image findings of a bony fragment (3 points), lateral shift of the patella (1 points), and trochlear dysplasia (2 points). A threshold score of 6.5 was determined using the area under receiver operating characteristic curve of 0.893 (p < 0.0001). In logistic regression analysis, a score of ≥7 was shown to be a dominant factor for recurrence (OR = 27.1, p < 0.0001). Furthermore, the association between the score and recurrence risk was as follows: a score of ≤4 = low risk (1/21 cases, 5%); a score of 5-7 = medium risk (13/27 cases, 48%); a score of ≥8 = high risk (30/33 cases, 91%). CONCLUSIONS: Based on the simple scoring system, patients who scored ≥8 were considered suitable for surgical treatment because of the high rate of recurrence, whereas those who scored ≤4 were considered suitable for conservative treatment because of the low rate of recurrence.

Alignment in the transverse plane, but not sagittal or coronal plane, affects the risk of recurrent patella dislocation.

PURPOSE: Abnormalities of lower extremity alignment (LEA) in recurrent patella dislocation (RPD) have been studied mostly by two-dimensional (2D) procedures leaving three-dimensional (3D) factors unknown. This study aimed to three-dimensionally examine risk factors for RPD in lower extremity alignment under the weight-bearing conditions. METHODS: The alignment of 21 limbs in 15 RPD subjects was compared to the alignment of 24 limbs of 12 healthy young control subjects by an our previously reported 2D-3D image-matching technique. The sagittal, coronal, and transverse alignment in full extension as well as the torsional position of the femur (anteversion) and tibia (tibial torsion) under weight-bearing standing conditions were assessed by our previously reported 3D technique. The correlations between lower extremity alignment and RPD were assessed using multiple logistic regression analysis. The difference of lower extremity alignment in RPD between under the weight-bearing conditions and under the non-weight-bearing conditions was assessed. RESULTS: In the sagittal and coronal planes, there was no relationship (statistically or by clinically important difference) between lower extremity alignment angle and RPD. However, in the transverse plane, increased external tibial rotation [odds ratio (OR) 1.819; 95% confidence interval (CI) 1.282-2.581], increased femoral anteversion (OR 1.183; 95% CI 1.029-1.360), and increased external tibial torsion (OR 0.880; 95% CI 0.782-0.991) were all correlated with RPD. The tibia was more rotated relative to femur at the knee joint in the RPD group under the weight-bearing conditions compared to under the non-weight-bearing conditions (p < 0.05). CONCLUSIONS: This study showed that during weight-bearing, alignment parameters in the transverse plane related to the risk of RPD, while in the sagittal and coronal plane alignment parameters did not correlate with RPD. The clinical importance of this study is that the 3D measurements more directly, precisely, and sensitively detect rotational parameters associated with RPD and hence predict risk of RPD. LEVEL OF EVIDENCE: III.

Correction to: Alignment in the transverse plane, but not sagittal or coronal plane, affects the risk of recurrent patella dislocation.

The author would like to correct the errors in the publication of the original article. The corrected detail is given below for your reading.

Extrinsic Factors as Component Positions to Bone and Intrinsic Factors Affecting Postoperative Rotational Limb Alignment in Total Knee Arthroplasty.

BACKGROUND: This study aimed to identify the factors affecting postoperative rotational limb alignment of the tibia relative to the femur. We hypothesized that not only component positions but also several intrinsic factors were associated with postoperative rotational limb alignment. METHODS: This study included 99 knees (90 women and 9 men) with a mean age of 77 ± 6 years. A three-dimensional (3D) assessment system was applied under weight-bearing conditions to biplanar long-leg radiographs using 3D-to-2D image registration technique. The evaluation parameters were (1) component position; (2) preoperative and postoperative coronal, sagittal, and rotational limb alignment; (3) preoperative bony deformity, including femoral torsion, condylar twist angle, and tibial torsion; and (4) preoperative and postoperative range of motion (ROM). RESULTS: In multiple linear regression analysis using a stepwise procedure, postoperative rotational limb alignment was associated with the following: (1) rotation of the component position (tibia: ß = 0.371, P < .0001; femur: ß = -0.327, P < .0001), (2) preoperative rotational limb alignment (ß = 0.253, P = .001), (3) postoperative flexion angle (ß = 0.195, P = .007), and (4) tibial torsion (ß = 0.193, P = .010). CONCLUSION: In addition to component positions, the intrinsic factors, such as preoperative rotational limb alignment, ROM, and tibial torsion, affected postoperative rotational limb alignment. On a premise of correct component positions, the intrinsic factors that can be controlled by surgeons should be taken care. In particular, ROM is necessary to be improved within the possible range to acquire better postoperative rotational limb alignment.

Increasing incidence of fracture and its sex difference in school children: 20 year longitudinal study based on school health statistic in Japan.

BACKGROUND: Studies on the epidemiology of pediatric fractures have been scarce in recent years although fractures are very common in childhood. Boys have a higher incidence of fractures than girls. Currently, societal trends have seemed to influence the difference in activity patterns between boys and girls, but the sex difference regarding longitudinal changes in fracture incidence is not well known. METHODS: We analyzed the school accident report in Niigata city, Japan and compared the incidence of fractures in elementary and junior high school students and the sex-related risk ratio between two 9-year periods separated by 20 years from their start and end points (1999-2007 and 1979-1987). RESULTS: The study included 383,273 students from 1999 to 2007 and 561,109 students from 1979 to 1987. Comparing these periods, the fracture incidence increased significantly by 2.4 times in boys vs 2.1 times in girls from elementary school and by 2.2 times in boys vs 2.9 times in girls from junior high school (all p ï¼œ 0.001). The sex-related risk ratio of boys to girls increased significantly from 1.47 to 1.64 in elementary school students. In contrast, it decreased significantly from 3.29 to 2.52 in junior high school students and the change was markedly significant because of the drastic increase in fracture incidence in junior high school girls. CONCLUSIONS: The reasons proposed for the increase in schoolchildren's fractures were an improvement in diagnosis owing to social background and increased participation in sports activities despite the general decline in children's physical fitness and exercise ability. In junior high school girls, in particular, there was an increase in fracture risk due to increased participation in sports activities.

Association between anteroposterior laxity in mid-range flexion and subjective healing of instability after total knee arthroplasty.

PURPOSE: Flexion instability following total knee arthroplasty (TKA) is a common indication of early revision. The association between the objective anteroposterior (AP) laxity direction in mid-range flexion and the subjective healing of instability remains unclear; thus, this study aimed to clarify this association. METHODS: In this study, 110 knees (74 females, 92 knees; 16 males, 18 knees) with medial pivot implants were examined with a median age of 79 (range 60-92) years for a median follow-up duration of 22 (range 6-125) months. AP laxity was measured using a KT-1000 arthrometer. Self-reported knee instability score was used for the subjective healing of instability. RESULTS: Eighty-seven knees did not feel unstable (Group 0), whereas 23 knees felt unstable (Group 1). There was a significant difference in AP displacement [Group 0: median 6 mm; range 2-15 mm and Group 1: median 8 mm; range 4-14; p < 0.0001]. The threshold value of 7 mm was determined using the area under receiver operating characteristic curve of 0.79 [95% confidence interval (CI) 0.69-0.88, p < 0.0001]. In multivariate analysis, AP displacement of ≥7 mm was an independent risk factor for feelings of instability (odds ratio 7.695; 95% CI 2.306-25.674; p = 0.001). CONCLUSIONS: AP laxity of ≥7 mm represents a known cause of feelings of instability. By controlling AP laxity in TKAs, without stiffness in the knee, it is possible to prevent feelings of instability. The clinical relevance is that AP laxity of <7 mm is one of the target areas in TKA. LEVEL OF EVIDENCE: IV.

Sex differences in femoral deformity determined using three-dimensional assessment for osteoarthritic knees.

PURPOSE: To characterize femoral deformities and determine sex differences in varus knee osteoarthritis (OA), femoral morphology and limb alignment were evaluated by using three-dimensional (3D) assessment, comparing healthy, elderly volunteers with osteoarthritic knees. METHODS: A total of 178 lower limbs of 169 subjects with knee osteoarthritis (136 women, 33 men; mean age 74.9 ± 5.2 years) and 80 lower limbs of 45 healthy, elderly subjects (24 women, 21 men; mean age 65 ± 4.9 years) were examined. A 3D extremity alignment assessment system was used to examine the subjects under weight-bearing conditions on biplanar long-leg radiographs using a 3D-to-2D image registration technique. The evaluation parameters were (1) femoral bowing in the coronal plane, (2) femoral bowing in the sagittal plane, (3) femoral neck anteversion, (4) hip-knee-ankle angle, and (5) femoral torsion. RESULTS: Higher femoral lateral bowing and slightly higher femoral internal torsion in the proximal diaphysis were observed in women with OA compared with healthy subjects. No difference in the higher varus malalignment, no alteration in the femoral anterior bowing, and no difference in the lower femoral neck anteversion were found between men and women when comparing healthy and OA subjects. CONCLUSIONS: The higher femoral lateral bowing and slightly higher femoral internal torsion in the proximal diaphysis in women are possibly a structural adaptation to mechanical use. The clinical significance is that the femoral deformities and the sex differences in knee OA have the potential to improve the understanding of the aetiology of primary varus knee OA. LEVEL OF EVIDENCE: IV.

External torsion in a proximal tibia and internal torsion in a distal tibia occur independently in varus osteoarthritic knees compared to healthy knees.

INTRODUCTION: The relative torsional angle of the distal tibia is dependent on a deformity of the proximal tibia, and it is a commonly used torsional parameter to describe deformities of the tibia; however, this parameter cannot show the location and direction of the torsional deformity in the entire tibia. This study aimed to identify the detailed deformity in the entire tibia via a coordinate system based on the diaphysis of the tibia by comparing varus osteoarthritic knees to healthy knees. METHODS: In total, 61 limbs in 58 healthy subjects (age: 54 ± 18 years) and 55 limbs in 50 varus osteoarthritis (OA) subjects (age: 72 ± 7 years) were evaluated. The original coordinate system based on anatomic points only from the tibial diaphysis was established. The evaluation parameters were 1) the relative torsion in the distal tibia to the proximal tibia, 2) the proximal tibial torsion relative to the tibial diaphysis, and 3) the distal tibial torsion relative to the tibial diaphysis. RESULTS: The relative torsion in the distal tibia to the proximal tibia showed external torsion in both groups, while the external torsion was lower in the OA group than in the healthy group (p < 0.0001). The proximal tibial torsion relative to the tibial diaphysis had a higher external torsion in the OA group (p = 0.012), and the distal tibial torsion relative to the tibial diaphysis had a higher internal torsion in the OA group (p = 0.004) in comparison to the healthy group. CONCLUSION: The reverse torsional deformity, showing a higher external torsion in the proximal tibia and a higher internal torsion in the distal tibia, occurred independently in the OA group in comparison to the healthy group. Clinically, this finding may prove to be a pathogenic factor in varus osteoarthritic knees. LEVEL OF EVIDENCE: Level Ⅲ.