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Background The learning curve of total hip arthroplasty (THA) using the anterior approach in the supine position is considered to be long owing to limited surgical field visualization. This study aimed to investigate the learning curve of a trainee’s cup placement using computed tomography (CT)-based navigation, focusing on identifying the number of cases required to achieve proficiency. Methods This retrospective cohort study included 112 hips that underwent total hip arthroplasty using CT-based navigation via anterolateral approach in the supine position by a trainee surgeon who had performed fewer than 20 prior THA cases. The absolute differences in cup alignment between postoperative measurements and intraoperative records, and differences in three-dimensional position between postoperative measurements and preoperative plan were assessed using the Mann–Whitney U test. A receiver operating characteristic (ROC) curve was used to determine the cutoff point for achieving accuracy. Results The absolute differences were 2.4°±2.5° (inclination) and 2.4°±2.0° (anteversion), and 1.5 mm ± 1.3 mm (coronal plane) and 1.4 mm ± 1.2 mm (axial plane). The ROC curve identified 20 cases as the cutoff point for proficiency, with significant improvement in cup inclination accuracy. Multivariate analysis revealed that the first 20 cases (odds ratio, 10.4; 95% confidence interval, 3.1−34.5) were a predictive risk factor for inaccurate cup alignment. No cup revisions or dislocations occurred. Conclusions This is the first study to identify the learning curve of cup placement using CT-based navigation via an anterior approach by an inexperienced trainee surgeon. Proficiency in cup placement was achieved after 20 cases using CT-based navigation.

Background Among the psychological factors, personality influences activities of daily living (ADL) in patient-reported outcomes (PROMs) following total knee arthroplasty (TKA). However, traditional personality assessments are often lengthy and challenging to implement in clinical settings. Although the Ten-Item Personality Inventory (TIPI) can be administered quickly, its use in patients undergoing TKA remains unknown. This study aimed to explore the effect of personality traits measured using the TIPI on ADL in PROMs at one and six months post-TKA. Methods A cohort of 140 individuals undergoing bilateral or unilateral TKA was enrolled. The ADL subscale of the Knee Injury and Osteoarthritis Outcome Score (KOOS-ADL) and TIPI, which measures five traits (agreeableness, conscientiousness, extraversion, openness, and neuroticism), were evaluated one month preoperatively and one and six months postoperatively. Changes in the KOOS-ADL over time were assessed with a mixed-effects model for repeated measures, including personality traits that were significantly correlated with the KOOS-ADL as fixed effects. Participants were classified into two groups according to their neuroticism scores, following the study hypothesis. Group differences in recovery of the KOOS-ADL trajectory was also analyzed by a mixed-effects model. Results Of the 140 patients, 104 completed follow-up. Neuroticism was significantly correlated with the KOOS-ADL in all phases. The KOOS-ADL significantly improved one and six months after surgery compared with that at baseline. Higher neuroticism scores were significantly associated with lower KOOS-ADL scores (estimate = -2.55, P  = 0.001). Patients in the neuroticism group also showed significantly lower KOOS-ADL scores than those in the non-neuroticism group (estimate = -5.56, P  = 0.03), although the group-by-time interaction was not significant. Conclusions The TIPI is a useful tool for assessing personality traits preoperatively, especially neuroticism, which influence ADL in PROMs. Patients with high neuroticism may benefit from attentive follow-up that acknowledges the potential gap between subjective and objective outcomes. Clinical trial number Not applicable

Purpose There is a lack of robust evidence for the magnitude of the effects of posterior capsular release (PCR) on intraoperative component gaps during total knee arthroplasty (TKA). The purpose of the present study was to quantify and compare the effects of partial versus full PCR on the intraoperative component gaps at various degrees of flexion during posterior-stabilized TKA. Methods Full PCR was performed on 39 consecutive cases (full PCR group), and partial PCR (the medial side up to and including the intercondylar notch) was performed on the subsequent 39 consecutive cases (partial PCR group) during posterior-stabilized TKA using the measured resection technique for varus knee osteoarthritis. Medial component gaps and varus angles at 0°, 10°, 45°, 90°, and a maximum of flexion were measured with a tensor device before and after the PCR. Differences between the two groups in post-release medial component gap increase and post-release joint varus angle increase were assessed using a t test. Pre- to post-release medial component gaps and joint varus angles in each group were compared using a paired samples t test. Results In both groups at 0° and 10° of flexion, post-release medial compartment gaps were significantly greater than pre-release gaps (all P  < 0.001). At 45°, 90° and maximum flexion, the medial compartment gap increase did not exceed the minimal detectable change in either group. At 0° and 10° of flexion, there was no significant difference in post-release medial compartment gap change between the two groups. In the full PCR group, post-release joint varus angles at 0° of flexion were significantly greater than pre-release angles ( P  < 0.001), while there was no significant pre- and post-release difference in the partial PCR group. The change in post-release joint varus angles at 0° of flexion was significantly greater in the full PCR group than in the partial PCR group. Conclusion Both full and partial PCR show similar clinical usefulness for increasing the medial component gap at extension and reducing component gap mismatch. A partial PCR can be used to avoid increasing joint varus angles at 0° of flexion. Level of evidence Level 2 (prospective comparative study).

Purpose Both coronal and sagittal laxity of well-functioning knees after total knee arthroplasty (TKA) was examined, and the correlations between the joint laxity and the clinical outcomes were analyzed to clarify the adequate joint laxity for the prosthesis, and the relationship between the laxity and the outcomes. Methods Forty well-functioning TKA knees with a high-flexion posterior-stabilized (PS) prosthesis were studied. All patients were diagnosed as having osteoarthritis with varus deformity and were followed up for 2 years or more. The coronal and sagittal laxity was assessed at extension and flexion, and the correlations between the joint laxity and the clinical outcomes were evaluated. Results The varus and valgus laxity averaged 5.6 ± 1.8° and 3.6 ± 1.2° at 10° knee flexion, and 7.4 ± 5.1° and 3.6 ± 2.7° at 80° knee flexion, respectively, and the AP laxity at 30° and 75° knee flexion averaged 8.7 ± 3.6 mm and 6.6 ± 2.3 mm, respectively. Knee flexion angle correlated with the joint laxity, while the other outcomes including patient-reported pain and instability were adversely affected by the greater laxity. Conclusions This study exhibited the importance of consistent medial laxity both at extension and flexion, which averaged 3.6°. Care should be taken to maintain the medial stability and to obtain adequate laxity both at extension and flexion during surgery. A few degrees of medial tightness can be allowed to achieve excellent clinical results after TKA for preoperative varus knees. Level of evidence Therapeutic study, Level III.

Background Developmental dysplasia of the hip causes secondary osteoarthritis. Finite element analysis suggests high hip joint contact pressure in patients with hip dysplasia and a reduction in contact pressure after periacetabular osteotomy. However, few biomechanical studies have examined the load distribution in the hip joint. This study aimed to investigate the biomechanical properties of load distribution in porcine hip joints at different acetabular coverages. Methods Six porcine hip joints were analyzed using three models: 1) neutral coverage, 2) 15° under-coverage (defined as dysplasia model), and 3) 15° over-coverage created by varying the acetabular coverage. The load distribution was assessed using a pressure-mapping sensor system after applying a loading force of 100 N to the hip joint. Results In the dysplasia model, the load was concentrated at the acetabular rim; in the neutral and over-coverage models, it was dispersed. The average contact pressure was significantly higher in the dysplasia model than in the neutral coverage model ([0.42 vs. 0.3 MPa]; p  = 0.004). The contact area was significantly smaller in the dysplasia model than in the neutral coverage model ([250.7 vs. 345.0 mm 2 ]; p  = 0.004). No significant differences were observed in contact pressure or area between the neutral and over-coverage models. Conclusions Insufficient acetabular coverage in the dysplasia model demonstrated higher contact pressure and smaller contact area than the neutral model. Conversely, the contact pressure and area in the over-coverage model did not differ significantly from those in the normal model. Therefore, surgeons should note that acetabular coverage overcorrection has limited effect; normalization is crucial during periacetabular osteotomy.

Nerve palsy following total hip arthroplasty (THA) can have a serious effect on a patient`s functional prognosis and on cost-effectiveness, and it is the leading cause of THA-associated medical litigation. However, only a few studies focus on femoral nerve palsy (FNP) following THA with the direct anterior approach (DAA). Moreover, several studies have reported that THA with DAA may result in higher complication rates, particularly during the so-called 'learning-curve period' for the surgeon. This study aimed to identify the incidence of FNP following primary THA with DAA, to determine presumed etiologies through a retrospective investigation of FNP clinical courses following primary THA with DAA and to identify any relationship between the occurrence of FNP following primary THA with DAA and the surgeon's experience of DAA. Since August 2007, DAA for primary THA was introduced in our institution. All 273 consecutive primary THAs with DAA (42 bilateral and 189 unilateral cases) between August 2007 and February 2014 were included in this study. All patients' charts and radiographs were reviewed to identify cases with palsy and to retrieve related factors. In this study, FNP was defined as weakness of the quadriceps femoris (manual muscle test <3) with or without sensory disturbance over the anteromedial aspect of the thigh. The incidence of FNP following primary THA with DAA was 1.1% (3/273 joints). In all 3 cases, the motor deficit recovered completely within a year. Suspected causes of the palsy in the 3 cases were believed to be improper positioning of the anterior acetabular retractor, excessive leg lengthening, or unknown etiology. There was no significant relationship between palsy and surgeon's experience of DAA. In THA with DAA for patients requiring major leg lengthening, the likelihood of FNP must be considered. To prevent FNP, the anterior acetabular retractor must be placed properly.

Background In this study, we aimed to compare the mid-term clinical and radiographic outcomes of highly porous and hydroxyapatite (HA)-coated porous titanium cups using the same cementless cup system in patients undergoing total hip arthroplasty (THA). We investigated the differences in radiolucent line (RLL) formation, cup migration, and clinical outcomes between the two surface processing methods. Methods This retrospective study included 209 hips (116 in the highly porous group and 93 in the HA porous group) with a minimum follow-up period of over 4 years. Using standardized anteroposterior pelvic radiographs, we conducted radiographic evaluations to assess cup orientation, initial gaps, and RLLs. Migration was analyzed using Einzel-Bild-Rontgen Analysis (EBRA), focusing on a subset of 28 randomly selected cups from each group as determined by power analysis. Clinical outcomes were measured using the Japanese Orthopedic Association (JOA) hip score. The primary outcome was the incidence of cup revision, and the secondary outcomes were postoperative JOA scores and the incidence of initial gaps and RLLs. Results No cup revisions were made in either group. Both groups had significant postoperative improvements in the JOA scores, with no differences observed during the follow-up period. At baseline, the initial gaps were observed at similar rates (highly porous vs. HA porous [51% vs. 52%]; p  = 0.97). However, at the 5-year follow-up, the initial gaps were resolved in nearly all cases. In the highly porous group, RLLs were significantly more frequent during the first-year follow-up (23% vs. 2%; p  < 0.0001). However, they decreased in all cases, and none of them exhibited RLL expansion. EBRA migration analysis revealed no significant differences between the groups. Conclusion Both groups demonstrated comparable mid-term clinical and radiographic outcomes. The highly porous group exhibited higher initial RLL incidences. However, RLL diminished over time without compromising the migration or clinical results. Both designs are reliable options for cementless THA.

The risk of insufficiency fractures at the iliac crest following pin insertion during robot-assisted total hip arthroplasty (THA) is unknown, as there have been very few reports on this complication. Here, we report two cases of insufficiency fractures of the contralateral iliac crest following robot-assisted THA using the Mako system (Stryker Orthopaedics, Mahwah, NJ, USA). Both patients underwent left THA using the anterolateral supine approach, and three threaded bone pins (4.0 mm diameter) were inserted into the right iliac crest for pelvic array fixation. In case one, all three pins achieved bicortical fixation. In case two, one pin demonstrated long transcortical fixation with the outer cortex of the ilium, another was inserted into soft tissue, and the third pin was fixed monocortically. Postoperatively, both patients were discharged without pain or radiographic evidence of fracture; however, contralateral iliac pain developed approximately four weeks postoperatively without trauma. Insufficiency fractures of the iliac crest at the pin insertion sites were confirmed by plain radiography. Bone union was observed within three to six months of conservative treatment in both cases, with T-cane ambulation and no weight-bearing restrictions. These cases suggest that both bicortical and transcortical pin fixation to the iliac crest may cause insufficiency fractures of the iliac bone. This report highlights the need for increasing awareness of insufficiency fractures associated with pin insertion in robot-assisted THA.

Background Post-operative tensor fascia lata (TFL) muscle atrophy due to superior gluteal nerve (SGN) injury during total hip arthroplasty (THA) can affect patients’ post-operative hip function. This study aimed to determine the incidence of TFL muscle atrophy in THA performed via the modified Watson-Jones anterolateral approach and the risk factors for TFL atrophy. Methods We reviewed pre- and post-operative magnetic resonance imaging (MRI) data of 164 patients who underwent cementless THA via the modified Watson-Jones approach at one institution. TFL atrophy was defined as worsening of ≥ 2 grades in the Goutallier classification or > 40% decrease in the cross-sectional area (CSA) of the TFL on post-operative MRI compared to that on preoperative MRI. Patients’ backgrounds were compared between those with or without TFL atrophy to determine the risk factors of TFL atrophy. Fatty atrophy grade and CSA of the gluteus minimus and medius were also evaluated. Results Thirteen (8.0%) cases of TFL atrophy were detected. The mean body mass index (BMI) in the cases with TFL atrophy was significantly higher than in those without TFL atrophy ( p  = 0.012). The fatty atrophy grade was worse post-operatively than preoperatively; moreover, the CSA of the gluteus minimus decreased. Conclusions We found a low incidence of TFL atrophy due to SGN injury after THA using the modified Watson-Jones approach. High BMI can be a risk factor for nerve injury. The gluteus minimus can be injured directly during surgery. We suggest that overexposure of the surgical site should be avoided, especially in patients with high BMI.

Background This prospective study aimed to clarify whether this novel device can evaluate the cup orientation during total hip arthroplasty (THA) more closely to that measured in postoperative computed tomography (CT) compared to the surgeon’s estimate using a manual goniometer. Methods We prospectively performed 30 cementless THAs via the anterolateral approach in supine position between October 2018 and July 2019, wherein cup orientation was evaluated by both a portable imageless navigation system (HipAlign) and a manual goniometer during surgeries. Primary outcome was the absolute estimate error [the absolute value of the difference between cup angles measured by postoperative CT and those measured by HipAlign (group H) or surgeon’s estimate using the manual goniometer (group S) during surgery]. The number of outliers of the absolute estimate error (> 10°) in each group was also estimated. Results The absolute estimate error of cup inclination in groups H and S was 3.3° ± 2.7° and 3.0° ± 2.5°, respectively ( p  = 0.51), whereas that of cup anteversion was 3.8° ± 3.4° and 6.0° ± 3.7°, respectively ( p  = 0.0008). The number of outliers of the estimate error in groups H and S was one case (3.3%) and six cases (20.0%), respectively ( p  = 0.04). In all six outlier cases, surgeons underestimated cup anteversion during surgeries. Conclusions This portable imageless navigation system was a useful method, especially for avoiding incorrect cup anteversion. Underestimation of cup anteversion during THA in the supine position with the conventional alignment assisting device should be given attention.