The authors prospectively studied a consecutive series of 41 patients who received PFA combined with TTT and PFA alone by the senior surgeon between 2012 and 2017. All patients had provided written informed consent for their participation in the study, which was approved in advance by our institutional reviewboard.
The inclusion criteria were as follows: (1) the age of patient was between 50 and 70; (2) patients who had a history of noninflammatory IPA with persistent pain and impairment despite conservative treatment; (3) patients with cartilage damage of greater than grade II; (4) patients with patellar subluxation or dislocation; (5) patients with an increased TT-TG (>20mm); (5) patients with a large Q-angle (>20°). The exclusion criteria were the presence of (1) tibiofemoral osteoarthritis; (2) systemic inflammatory arthropathy; (3) post-traumatic osteoarthritis; (4) patella baja; (5) psychogenic pain; (6) concomitant cruciate ligament or collateral ligament injury.
Among our 41 patients, 2 patient was lost, and 3 others met one or more exclusion criteria, leaving 36 patients (31 women and 5 men) aged 61.1 ± 7.3 years with complete data for this study. A total of 17 patients underwent patellofemoral arthroplasty combined with tibial tubercle transfer, and 19 patients underwent patellofemoral arthroplasty only. The Consolidated Standards of Reporting Trials (CONSORT) fiowchart showing the selection of patients is shown in Figure 1. All eligible patients had CT scans preoperatively and at 12 months follow-up, to assess the stability of the patellofemoral joint on axial slices. In addition, the demographic and clinical features of all the patients were asked. Balance was assessed with the single leg stance test (SLST) and timed get up and go (TGUG), and functionality was evaluated with stair climbing test (SCT) and the Western Ontario and McMaster Universities Osteoarthritis Index score (WOMAC).
All patients received the Avon Patellofemoral prosthesis (Stryker Orthopaedics, Mahwah, New Jersey) (Figure 2). The procedure of PFA was performed in a manner consistent with previously published reports [32, 33]. The critical principles in successful patellofemoral arthroplasty for the treatment of isolated patellofemoral arthritis are as follows: the use of a muscle-sparing approach, appropriate placement of the femoral cutting guide (avoidance of retroversion or internal rotation), careful attention to avoid femoral notching, avoiding overtensioning of the patellofemoral retinaculum (through repairing the patella and releasing tension on the lateral retinaculum), patellar medialization (to avoid medial impingement), and careful soft tissue balancing.
The following are the main principles of successful tibial tubercle transfer for the treatment of patellofemoral arthritis proposed by Saleh et al. in 2005  (Figure 2): (1) restoration or maintenance of the proper balance of the extensor mechanism; (2) transfer of a painful and degenerated area to a less loaded articular surface and reduction of the load on the lateral aspect of the patella through medial transfer of the tibial tuberosity; (3) relief of pain caused by the patellar retinaculum.
The diagnosis of isolated patellofemoral arthritis was confirmed by preoperative radiographs (in anteroposterior, lateral and merchant views) (Figure 3). The diagnosis of patellar dislocation was confirmed by a patellar apprehension test (>1.5cm)  and CT of the patellofemoral joint with the non-weight-bearing knee in full extension  (Figure 4). Arthroscopic assessment was used to identify chondral lesions and concomitant pathology before performing operative techniques in all patients.
All patients underwent CT to assess the stability of the patellofemoral joint, preoperatively and at final follow-up, on a particular axial image which was established at the point with the greatest epicondylar width based on measurements on axial slices [36, 37]. All data were measured using Sante DICOM Viewer Free (64-bit) verson 5.2 (Santesoft, Inc. Athens, Greece), which has an accuracy of 0.01° for angles and 0.01 mm for distance . In order to minimize errors of measurement, all measurements were performed under the same conditions by two authors (YW and GY). After an interval of three weeks, one measured the 36 samples again and the intra and interobserver reliabilities were determined using intra-class correlation coefficients (ICCs).
SLST was used to evaluate static balance. The patients were instructed to stand on one foot and bend the other leg from hip and knee. This leg should not touch the other leg, and the balance is held for as long as possible. The test was performed by the affected leg with three repeats and the highest performance was recorded. If the bended leg touches the supportive leg or the foot touches the floor or the arms get support from anywhere, the test is failed [39, 40]. TGUG is a test used to assess dynamic balance. For the test, the person is asked to sit and stand up from a standard chair and walk a distance of approximately 3 metres, turn around and walk back to the chair and sit down again. The time begins with the instruction and ends with sitting down again. We repeated the test 2 times and found the mean value. The values below than 20 s are within normal limits for transfer and mobility; however, the values greater than 30 s means that the risk of dependence and falling is increased, and the elderly person needs assistance during daily activities and uses assistive devices for ambulation [39, 41].
In SCT, the patients were asked to climb up and down the ten step stairway, and this involves alternatively placing climbing up and down as fast as possible onto a step that was 19 cm high and 27 cm deep. We repeated the test two times to prevent tiredness and found the mean value [39, 41, 42]. In addition, the Western Ontario and McMaster Universities Osteoarthritis Index score (WOMAC; 0 to 100, 0 being the best score) [25, 43] was also collected to evaluate the knee functionality.
Sample size was calculated by Raosoft, Inc. Minimal clinically important difference was considered 15 % of maximal score for WOMAC for Total knee arthroplasty (TKA) by Escobar et al. [39, 44]. With a change 15 % in WOMAC score, the estimated sample size required to detect a statistically significant difference between groups, at a 10 % significance level with a power of 90 %, was 35 patients.
Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS) version 16.0 (SPSS, Chicago, Illinois). The Kolmogorov–Smirnov test was used to test the normality of numerical data. Levene’s test was used to assess the homogeneity of the data. All numerical variables showed a normal distribution or equal variance. Differences between the two groups were analysed with a two-sample Student’s t-test. The differences of gender (female/male), leg (right/left) and the apprehension sign were analyzed using Pearson’s chi-squared test. Numerical data are shown as mean and standard deviation, and categorical data as numbers with percentages. A p-value of < 0.05 was considered statistically significant.