Good Operative Effects of Patellofemoral Arthroplasty Combined With the Tibial Tubercle Transfer for Isolated Patellofemoral Arthritis Patients With an Increased Tibial Tuberosity-trochlear Groove Distance

Purpose: The objective of present study was to investigate the operative effect of patellofemoral arthroplasty combined with the tibial tubercle transfer for isolated patellofemoral arthritis patients with an increased tibial tuberosity-trochlear groove distance (>20mm). Methods: A prospective study was performed between November 2012 and December 2017. Finally, thirty-six cases, with a mean age of 61.1 ± 7.3 years, were admitted to our study. A total of 17 patients underwent patellofemoral arthroplasty combined with tibial tubercle transfer, and 19 patients underwent patellofemoral arthroplasty only. 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. Knee balance was assessed with the single leg stance test and timed get up and go, and functionality was evaluated with stair climbing test and the Western Ontario and McMaster Universities Osteoarthritis Index score. (P>0.05) Results: Preoperatively, the data regarding the knee balance and functionality were not signicantly different between the two Groups (P>0.05). Many measurements showed signicant differences between the two groups at the last follow-up. Signicant differences were seen in well-known measurements such as the SLST (Group I, 28.9 s (SD 7.5); Group II, 20.3 s (SD 5.9); p < 0.001), TGUG (Group I, 13.4 s (SD 3.2); Group II, 16.8 s (SD 3.1); p < 0.001), and SCT (Group I, 18.6 s (SD 6.8); Group II, 24.5 s (SD 8.7); p < 0.001). Additionally, the median WOMAC score was improved from 62.7 (SD 11.2) preoperatively to 25.7 (SD 8.2) one year postoperatively in Group I and from 64.1 (SD 10.7) to 36.2 (SD 9.7) in Group II, which were


Introduction
Isolated patellofemoral arthritis (IPA) is a common condition affecting the middle-aged and the elderly [1].
It is estimated that approximately 10% of patients older than 40 years presenting with knee pain have radiographic evidence of IPA [1]. Isolated degeneration of the patellofemoral articulation has also been reported in 24% of women and 11% of men greater than 55 years who present with symptomatic knee arthritis [2]. When looking at patients older than 60 years, the disease was present in 34% of women and 19% of men according to some studies [1,3]. Noble and Hamblen [3] reported patellofemoral osteoarthritis in 79% of 100 cadavers aged ≥65 years. Therefore, given the prevalence of this disease, we should attach great importance to the treatment of IPA.
Furthermore, the importance of trochlear dysplasia as a major predisposing factor for patellar dislocation has been recognized for many years [16,17]. Fithian et al. [16] and Dejour et al. [18] reported that 85% and 96% of patients with patellar dislocation, respectively had trochlear dysplasia. Thus, IPA is not rare associated with patellar dislocation.
Niu et al. [19] have proven that early patellar dislocation not only results in eventual patellofemoral arthritis but also can lead to tibial tubercle lateralization and an increased tibial tuberosity-trochlear groove distance (TT-TG). The tibial tubercle, attached to the patellar tendon, is an important structural bone marker in terms of the Q angle and TT-TG, which re ect limb alignment. Its position affects the convolution of the patellofemoral joint. Lateralization of the tibial tuberosity can cause patellar dislocation; if lateralization of the tibial tubercle increases, then the Q angle, TT-TG, and external vector force of the patella also increase [19]. Most studies have con rmed that externalization of the tibial tubercle is a potential cause of patellofemoral instability [20][21][22][23][24].
The normal TT-TG distance measured on computed tomography (CT) scans is between 10 and 20 mm and more than 20 mm is the operative indication of tibial tubercle transfer (TTT) [18]. For elderly patients, the surgical "gold standard" for IPA has traditionally been patellofemoral arthroplasty (PFA), which literature has demonstrated to provide good long-term outcomes [25][26][27]. However, for IPA patients, there has been at least a 5% rate of patellar subluxation after PFA at a long-term follow-up [28]. This suggests a greater rate of dislocation in IPA patients with an increased TT-TG (>20mm) and a large quadricepsangle (Q-angle) (>20°) [18]. Recently, numerous studies suggested that one of the reasons for the failure of patellofemoral arthroplasty is the subluxation or dislocation of the patella [29][30][31]. Theoretically, for severe patellofemoral track deformity that cannot be corrected (such as abnormal large Q-angle and positive J-sign), the simple soft-tissue balancing procedures in patellofemoral joint replacement is often not able to achieve satisfactory results, and it is usually necessary to combine the technique of tibial tubercle transfer. To the authors' knowledge, there are no published studies that investigate the operative effect of PFA combined with TTT for IPA patients with an increased TT-TG (>20mm). Therefore, we believe that for such special patients, this combined operation may be more effective to avoid patellar instability and improve knee function than PFA alone. The purpose of this study was to investigate the operative effect of PFA combined with TTT for IPA patients with an increased TT-TG (>20mm).

Clinical background
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 nonin ammatory 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)

Operative technique
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.

Assessment
The diagnosis of isolated patellofemoral arthritis was con rmed by preoperative radiographs (in anteroposterior, lateral and merchant views) ( Figure 3). The diagnosis of patellar dislocation was con rmed by a patellar apprehension test (>1.5cm) [35] and CT of the patellofemoral joint with the nonweight-bearing knee in full extension [36] ( 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 nal 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 [38]. 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 coe cients (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 oor 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.

Statistical analysis
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 signi cant difference between groups, at a 10 % signi cance 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 twosample 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 signi cant.

Results
Demographic features of the groups were presented in Table 1. Statistically signi cant difference was not found between the two groups in comparison with demographic characteristics. The intra and inter observer intra-class correlation coe cients were high for all measurements (Table 2). Preoperatively, there were also no statistically signi cant differences between the patellofemoral joint stability in the two groups (n.s.) (Table 3). Additionally, all patients in both groups had a patellar lateral shift that exceeded 1.5 cm with a hard end point for the apprehension test, with no signi cant difference.
At the nal follow-up, these parameters were signi cantly different between the two groups, such as the Q-angle (Group I, 10. Group II, 13.72 mm (SD 2.41); p < 0.001) (Table 4). Additionally, 5 patients in Group II had a patellar lateral shift that exceeded 1.5 cm. However, all patients in Group I had a patellar lateral shift that less than 1.5 cm for the apprehension test, with a signi cant difference between the two groups.   Preoperatively, the data regarding the knee balance and functionality were not signi cantly different between the two Groups (n.s.) ( Table 5). Many measurements showed signi cant differences between the two groups at the last follow-up (Table 6). Signi cant differences were seen in well-known measurements such as the SLST (Group I, 28.9 s (SD 7.5); Group II, 20.3 s (SD 5.9); p < 0.001), TGUG (Group I, 13.4 s (SD 3.2); Group II, 16.8 s (SD 3.1); p < 0.001), and SCT (Group I, 18.6 s (SD 6.8); Group II, 24.5 s (SD 8.7); p < 0.001). Additionally, the median WOMAC score was improved from 62.7 (SD 11.2) preoperatively to 25.7 (SD 8.2) one year postoperatively in Group I and from 64.1 (SD 10.7) to 36.2 (SD 9.7) in Group II, which were also signi cantly different between the groups.

Discussion
Correlational studies noted better outcomes in IPA patients with subluxation and dislocation after PFA, which were commonly associated with trochlear dysplasia [25,26]. Marco Valoroso et al. [27] pointed out that PFA improves patellofemoral congruence by correcting trochlear dysplasia and standardizing radiological measurements as patellar tilt and TT-TG. However, for IPA patients, there has been at least a 5% rate of patellar subluxation after PFA at a long-term follow-up [28]. This suggests a greater rate of dislocation in IPA patients with an increased TT-TG (>20mm). In normal knees, the Q-angle is under 20°a nd the congruence-angle (CA) are close to 10° [36,45]. The normal TT-TG distance, patellar lateral shift (PLS) and patellar tilt angle (PTA) are under 20 mm, 15 mm and 20°, respectively [18,[46][47][48][49][50]. Our results showed that both of the two surgical methods could improve the stability of patellofemoral joint. But compared with PFA, the standardizing radiological measurements of this combined operation were more in line with normal knee level. In addition, there were ve patients with subluxation of the patella in Group II and none in Group I ( Figure 5). Therefore, for such special patient, the stability of the patellofemoral joint can be signi cantly improved by the PFA combined with TTT, compared with PFA alone, with decreased recurrent instability.
SLST was used to evaluate static balance. Vuorenmaa et al. [51] found that time standing on one foot was 10.3±14.7 s at the third month in unilateral TKA patients, which was a signi cant improvement over the preoperative evaluation results. In the study by Akbaba et al. [39], the SLST period increased from 3.5±4.5 s to 44.2±32.2 s for right leg and from 2.6±2.4 s to 42.6±32.5 s for left leg after bilateral TKA at the second month. In the current study, our data showed that the static balance of Group I is much higher than that of Group II after one year of surgery.
TGUG is a test used to assess dynamic balance. The values below than 20 s are within normal limits for transfer and mobility [39,41]. Zeni et al. [52] reported that the TGUG period decreased from 8.7±1.8 s to 6.7±1.3 s at the second year for bilateral TKA. In the study by Akbaba et al. [39], the TGUG period decreased from 21.7±11.2 s to 12.9±2.9 s by effective rehabilitation training after two months of bilateral TKA. In the present study, more improvement was obtained for dynamic balance in Group I when compared to Group II at the rst year after operation.
SCT is an important index to evaluate the function of knee joint and the number of stairs and application of SCT differ in the literatures. In the study by Akbaba et al. [39], it was found that the SCT (10 stairs) results were pre-operative 49.1±24.5 s, rst month 34.9±20.5 s, second month 17.5±7.2 s after bilateral TKA. Marcus et al. [53] found that the SCT (10 stairs) period was pre-operative 17.2±6.8 s and sixth week 9.2±3.9 s following the eccentric exercise programme in TKA patients. Zeni et al. [52] showed that the duration of SCT (12 steps) decreased from 16.8±5.7 s to 10.0±1.9 s in bilateral TKA patients. Heiberg et al. [54] reported that the SCT duration of 8 stairs was 14±5 s in the pre-operative period and was 6±2 s at the post-operative third month. In our study, the duration of SCT (10 steps) decreased from 45.6±17.8 s preoperatively to 18.6±6.8 s postoperatively in Group I and from 48.3±19.1 s to 24.5±8.7 s in Group II after one year of surgery. The results showed that with the combined operation of PFA combined with TTT, the climbing up and down period decreased signi cantly when compared to PFA alone.
In addition, the WOMAC score is also a widely accepted index of knee function evaluation. Kim et al. [55] demonstrated that WOMAC values decreased at the sixth month in the groups with or without passive range of motion (ROM) when compared to the pre-operative period. Akbaba et al. [39] reported that WOMAC pain, stiffness and function values decreased at the second month when compared to the pre-operative period in TKA patients. Walls et al. [56] and Rooks et al. [57] revealed a similar decrease in the WOMAC score. In our study, the median WOMAC score was improved from 62.7±11.2 preoperatively to 25.7±8.2 one year postoperatively in Group I and from 64.1± 10.7 to 36.2±9.7 in Group II. Our results showed that the WOMAC score of the combined surgery is better than that of PFA alone.
Through the above discussion, we can draw the following conclusions. Firstly, both of the two surgical methods could improve the stability of patellofemoral joint and function of knee joint when compared to the pre-operation. Secondly, it also shows that for such special IPA patients with an increased TT-TG (>20mm), the combined operation of PFA combined with TTT can better restore the involutional relationship of patellofemoral joint and further improve the balance and function of knee joint. Therefore, for such special IPA patients, this combined operation may be more effective to avoid patellar instability and improve knee function than PFA alone. The present study was approved by the Academic Ethics Committee of the Third Hospital of Hebei Medical University. The trial was conducted in accordance with the Declaration of Helsinki. All participants will be provided information about the study and enrolled into the trial if they meet all eligibility criteria and voluntarily sign an informed consent statement.

Consent for publication
Not applicable.

Availability of data and materials
The datasets used during the current study available from the corresponding author on reasonable request.    A) Diagnosis of patellar dislocation by transversal imaging of CT with the non-weight-bearing knee in full extension before operation. B) The normal TT-TG distance measured on computed tomography scans is less than 20 mm and more than 20 mm is the operative indication of tibial tubercle transfer.