This study has been approved by the Ethical Committee of the University of Witten/Herdecke (Study No. 108/2015) and was carried out in accordance with the ethical standards laid down in the Declaration of Helsinki. The participation was voluntary and all patients gave their informed consent to this study.

This study includes 33 knees with a severe trochlear dysplasia in 30 patients (m/f = 12/21, mean age 24 ± 9 years) who underwent surgery with a combination of trochleoplasty and MPFL reconstruction as described below. The estimated mean number of dislocations before surgery was 35 ± 24 (range: 1-250). Before surgery, all patients had a clear positive patella apprehension sign. Exclusion criteria were additional surgeries such as corrections of the knee rotation and/or axis, tibial tuberosity transfers, etc. Moreover, patients with advanced osteoarthritic changes or previous fractures of the knee were not included in this study. Factors of patellar instability were measured on preoperative radiographs and magnetic resonance imagings (MRIs) or CT scans[4]. In our series, the mean Caton-Deschamps index was 1.09 ± 0.09 and the tibial tubercle-trochlear groove distance was 16 mm ± 3 mm. The lateral trochlea inclination (LTI) angle was measured on the most superior MRI slice depicting the cartilage of the trochlear surface using the method described by Carrillon et al[17]. In our patients, the mean angle between the posterior contours of the condyles and the lateral facet of the trochlear groove was -9° ± 6°. Trochlear dysplasia was graded using the Dejour classification. Nine knees showed a type B, ten a type C and 14 a type D trochlear dysplasia.

The mean follow-up period was 29 ± 23 mo. Possible complications such as recurrent dislocations, fractures, knee stiffness, etc. were registered. In addition to a routinely performed clinical examination, the knee function was assessed with different scoring scales. In this study, the Kujala anterior knee pain scoring, the knee-specific outcome measure of the International Knee Documentation Committee (IKDC) and the Lysholm knee scale was used. Moreover, additional questionnaires and different visual analog scales were used to assess the general satisfaction of the patients, the satisfaction with the cosmetic outcome, the return to sports, a potential avoidance behaviour, different sports activities as well as a potential anterior knee pain.

The gracilis tendon is harvested through a minimal invasive posterior or anterior harvest over the pes anserinus. After whip-stitching the armed graft has been given in a Vancomycin solution for presoaking in most cases. In contrast to the Bereiter technique, which describes a lateral parapatellar approach, we prefer a short medial parapatellar access to expose the trochlea. A seven to 10 cm long skin incision running from the medial part of the patella to the distal part of the quadriceps tendon is made (Figure 2A). The medial patellar retinaculum is divided by leaving enough tissue near the patella for later reattachment. The proximal joint capsule is incised between the superomedial pole of the patella and the vastus medialis obliquus muscle. Finally, the distal quadriceps tendon is split. Using this distally shortened medial parapatellar approach, the saphenous nerve with its infrapatellar branch is usually not affected. With this simple arthrotomy and a dislocation of the patella towards lateral, the trochlea can be completely exposed (Figure 2B). A soft tissue tunnel between both layers and running down from medial patella to the femoral MPFL insertion can be created easily by using this approach. After exposure of the trochlea, the articular cartilage is separated from the synovium. Using a curved osteotome, a thin osteochondral flap, leaving 2 mm of subchondral bone, is very carefully chiselled off the medial and lateral trochlea (Figure 2B). The flap is extended down to the intercondylar notch. The trochlear bone is then deepened using chisels and a high-speed diamond burr (Figure 2C). The lateral edge of the trochlea is not lowered to obtain a raised lateral inclination angle with an effective bony stabilization. This way, a recentralized groove is created. In cases with an increased TT-TG distance, the groove can also be deepened in a lateralized position. Then, the thin and flexible osteochondral flap is pressed into the newly formed groove. When the flap is not flexible enough, it should be thinned with the high-speed burr. The osteochondral flap is fixed into the new formed groove with a transosseous resorbable 3 mm Vicryl band (Ethicon Products, Norderstedt, Germany). For this step, the Vicryl band is passed from the distal start of the formed groove, which is usually a couple of millimeters above the intercondylar notch, through the femoral condyle using a big and eyed curved needle (Figure 2D). A second, proximal transosseous passage through the femoral condyle is performed starting exactly at the proximal end of the new groove. After pressing the flap onto the femoral bone, the Vicryl band is tightened and fastened (Figure 2E). If needed, a second Vicryl band, running from the distal part of the sulcus to the craniolateral edge of the trochlea, can be used to press the flap more firmly to the bone. After preparation of the medial patella facet and a debridement of the bone between both layers, two guide wires, coming from the middle of the medial edge of the patella and the superomedial pole, are advanced into the bone. Using a 4.5 mm cannulated drill bit, a V-shaped tunnel with an angulation of around 120° is created. The drillings should run as vertically as possible and leave a bone bridge between both ends of at least 15 mm (Figure 2F). The starting points of both patellar tunnels may correspond to a recent anatomic study on the MPFL, where the attachment spreads out along the upper and middle third of the medial patella edge[18]. Using a curved needle, the graft is passed through the tunnel (Figure 2G). After palpation of the bony femoral MPFL insertion, the origin is marked with a K wire under fluoroscopic guidance. As described by Schöttle et al[19], the radiographic landmark of the femoral attachment is located 2.5 mm distal to the posterior origin of the femoral condyle and 1.3 mm anterior to a line at the posterior cortical bone of the femur (Figure 2H). After a stab incision, the guide wire is advanced to the opposite cortical bone of the femur and overreamed to a depth of at least 4 cm. Then the medial capsule is exposed and a soft tissue tunnel between both layers of the capsule is extended down to the femoral insertion of the MPFL graft (Figure 2I). Two beath pins are introduced into the femoral bone tunnel. From the end of this blind-ending tunnel, both pins are advanced in divergent directions and passed through the lateral cortical bone of the femur (Figure 2J). Using the prepared soft tissue tunnel between both layers of the medial capsule, both armed ends of the graft loop are shuttled from the patella down to the femoral bone tunnel. Each strand of the whipstitch sutures at both ends of the graft is passed through the eyelets of the beath pins. By passing the pins through the femur, the ends of the whipstitch sutures are passed through the lateral femoral cortex. By pulling on each suture pair, both armed ends of graft are introduced into the tunnel (Figure 2K). On the lateral femur, a temporary knot is tied down to the lateral femoral cortex in approximately 30° knee flexion. This knot is temporarily fastened with a needle driver or by hand. The tension of the MPFL graft and the achieved motion of the patella is assessed by knee flexion and lateral translation of the patella. If necessary, the tension of the double limbed graft can be adjusted and functionally tested again. Finally, the knot is permanently tied up by using a series of half hitches. The inner layer of the capsule underneath the graft and the thicker superior layer which lies above the graft are closed separately and right up to both layers at the patellar margin (Figure 2G and I). The tendon of the vastus medialis obliquus muscle can be tightened medially to the patella if needed. Finally, the longitudinal incision within the distal part of the quadriceps tendon is closed up.

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Figure 2 Step-by-step description of the combined surgical procedure. A: Short skin incision running from the medial part of the patella to the distal part of the quadriceps tendon; B: Using a curved osteotome, a thin osteochondral flap is chiselled off the trochlea; C: Trochlear bone is deepened using chisels and a high-speed burr; D: Vicryl band is passed from the distal beginning of the formed groove through the condyle using a curved needle; E: Starting at the junction between the trochlear cartilage and the anterior femoral bone, the other end of the band is passed through the femur. After pressing the flap onto the bone, the band is fastened; F: Drilling of a V-shaped tunnel coming from the superomedial pole and from the middle of the medial facet of the patella; G: Gracilis tendon is passed through the drill holes; H: Femoral medial patellofemoral ligament (MPFL) insertion side is marked with a K wire under fluoroscopic guidance; I: Prepared soft tissue tunnel between both layers of the medial retinaculum is running down to the femoral MPFL insertion; J: Two divergent beath pins are passed from the blind ending tunnel through the lateral femoral cortex; K: After passing the strands to the lateral femur, a temporary knot is tied down in approximately 30° knee flexion; L: Arthroscopy of the trochlear groove after trochleoplasty.

In the first two post-operative weeks, a partial load was recommended. In the first six weeks, an adjustable knee brace was prescribed. During the first two weeks, the motion was restricted to a knee flexion between 0° and 30°. During the third and fourth weeks, the flexion was limited to 60° and to 90° within the fifth and sixth weeks. In this context, we have to amend that we do not recommend these limitations any more (please see discussion). A rehabilitation of 12 wk was recommended before starting with sporting activities.

The statistical analyses in this study were done by using the SPSS statistics 22.0 software (SPSS Inc, Chicago, IL, United States). A statistical review of the study was performed by a biomedical statistician. The Wilcoxon signed-rank test was used to compare the pre- and postoperative clinical scores. For the comparison of the scoring between different groups within this study we used the Mann-Whitney-U test.

No patients showed signs of a persistent instability. Thus, persistent postoperative instabilities from subtle instability with anamnestic subluxations to frank dislocations were not reported. The apprehension tests were negative in all patients. Two patients reported a persistent avoidance behavior. Even if they had already mastered typical stress situations with experienced luxations in the past, they continued a slight avoidance behavior. Both patients reported that this behaviour was caused psychologically and not instability- and/or pain-related. All other patients reported being able to fully load their knee without anxiety.

In 31 of the 33 cases (94%) the patients were very satisfied with the clinical outcome of the surgery. All of them would again decide for the same procedure. Two patients (6%) were dissatisfied. Both patients had a flexion deficit between 20° and 30°. One of these patients also reported an anterior knee pain. However, the main reason for her disappointment was that she was not able to play and to crawl an all fours with her toddler.

Regarding the cosmetic outcome, all patients were satisfied with the outcome. Thus, no patients felt impaired in their self-confidence and/or in their clothing choices such as skirts or trousers. Two patients were only partially satisfied because they would have preferred a shorter and narrower scar. On the other hand, they would not decide for an aesthetic correction operation assuming that this would be most likely successful.

Four patients (12%) reported no changes of their pain symptoms, which could be described in all of these cases as a slight anterior knee pain occurring occasional during exercise and/or longer runs. A further four patients (12%) reported an increase of pain at the last follow-up. Further six patients (18%) reported a severe knee pain before surgery which improved almost completely at the last follow-up. Further 14 patients (42%), who mainly showed severe pain symptoms before surgery, became completely pain-free. Further five patients (15%) had no knee pain before surgery as well as at the last follow-up. Taken together, in 29 of 33 cases (89%), the patients either remained unchanged with mild symptoms or no pain, or showed a notable improvement to an almost or completely pain-free situation. The assessment of pain using a visual analog scale showed a significant pain reduction from a mean of 4.8 ± 2.0 to 1.3 ± 3.4 points (P < 0.0001).

In 25 of the 33 cases, patients were engaged in regular physical activity before surgery. After surgery, two of these did not return to full sporting activities. Both did not have any complaints. One patient reported an anxiety or avoidance behaviour for typical stress situations as a reason for not returning to his full sporting activities. He played soccer and he felt that this sport is too strenuous or risky for his knee. The other patient preferred to wait for the surgery of her contralateral knee. Of the eight patients who were not engaged in sporting activities before surgery, three moved to the group of patients engaging in regular physical activity. The remaining five patients were very satisfied with their knee function during daily activities and none of them reported any complaints or recurrent luxations. Nevertheless, four of them still had no interest in sporting activities. One of the five patients who remained uninvolved in sports reported a continuing avoidance behavior in her daily activities. Although she had already mastered typical stress situations with previously experienced dislocations, she reported persistent avoidance behavior.

No deep infection or wound infection occurred. Recurrent dislocations, patella fractures, breakages of the osteochondral flap during surgery etc, were not registered. The re-operation rate in our study was 6% (2/33). In both cases, an early postoperative motion deficit with a markedly reduced flexion between 70° and 90° was treated. After the hospital stay, both patients did not attend the recommended physiotherapy. In both cases, arthroscopy with an arthrolysis and a mobilization was performed around three months after the initial surgery. During arthroscopy, adhesions within the joint were removed. Arthroscopy of the trochlear groove and the cartilage of the patellofemoral joint showed regular findings, the vicryl fiber was fully resorbed (Figure 2L). Subsequently, both patients had better motion and decreased pain during motion. At the final follow-up, they showed an unlimited motion and no anterior knee pain. Furthermore, we have to report three patients with a slight flexion deficit at the last follow-up. Two showed a deficit of around 20°. Both patients reported a slight anterior knee pain occurring after prolonged load during physical exercise such as longer runs, etc. A persistent and/or severe anterior knee pain was denied. The third patient showed a flexion deficit of 35° compared to the contralateral side. He was pain-free.

As depicted, the Kujala score improved significantly from a mean of 64 ± 16 points preoperatively to 94 ± 9 points postoperatively (P < 0.0001). The Lysholm score improved significantly from a mean of 63 ± 17 to 95 ± 6 points and the IKDC score from 58 ± 11 to 85 ± 12 points, P < 0.0001, respectively. There were no significant differences regarding age, sex, BMI and affected side of the knee (P > 0.05).

Trochlear dysplasia is an important and frequent instability factor in patients with recurrent patella dislocations. These patients typically have an insufficiency of the medial patellofemoral ligament (MPFL), which is an additional instability factor. During a trochleoplasty, the articulation of the patella is changed to a more dorsomedial position, which might worsen the insufficiency of the medial soft tissue restraints. All these patho-biomechanical conditions are relevant for a symptomatic instability and should therefore be addressed during surgery. Despite its relevance, studies on combined concepts to address all pathological conditions are rare.

Regardless of whether a MPFL reconstruction is performed as an isolated surgery or as in combination with a trochleoplasty, an adequate tensioning of the graft is a key problem during these surgical procedures. Thus, an overtensioning might lead to stiffness, pain, cartilage degradation, arthrosis and patella fractures, whereas an undertensioning leads to persistent instability complaints. Authors’ technique, which provides an adequate graft tensioning, appears to be helpful to solve these problems. Thus, the study presented here was a necessary first step in exploring authors’ method, which seems to provide reasonable advantages.

To realize a combination of authors’ technique of a balanced MPFL reconstruction to the bony alignment procedure, authors modified both, the technique and especially the approach of the Bereiter trochleoplasty as well as authors’ recently published technique of a balanced MPFL reconstruction. To evaluate authors’ method, 33 knees with with recurrent patellar dislocations and a trochlear dysplasia were evaluated after a mean follow-up of 29 mo.

To assess the outcome of authors’ modified technique, the Kujala, IKDC and Lysholm scoring were evaluated. Moreover, patients’ satisfaction with the general outcome, the return to sports, the cosmetic outcome, the pre- and postoperative pain and a potential avoidance behaviour were assessed.

The preliminary clinical results of this technique demonstrate a good efficiency in relieving the symptoms and improving the function of the affected knee. There were no signs of a persistent instability. A significant pain reduction and a significant improvement at the Kujala, Lysholm and IKDC scoring is demonstrated. 94% of the patients were very satisfied with the clinical and cosmetic outcome of the surgery. 92% of the patients who were engaged in regular physical activity before surgery returned to full sporting activities. In regard to three patients showing a slight flexion deficit compared to the contralateral side at the last follow-up, authors changed the postoperative treatment protocol. Thus, authors do not limit the flexion any more post-operatively and authors try to ensure intensive motion exercises. One patient did not return to sports and another patient, who was not active in sports before surgery, still did not participate in regular physical activity after surgery. Both reported that this was related to a persistent avoidance behavior and not instability- and/or pain-related. This specific problem, which reduced authors’ return to sports rate, might highlight the need for an intensive individual sports therapy.

In patients with a symptomatic patellar instability, both a trochlear dysplasia and an insufficiency of the MPFL should be addressed during surgery. Regarding the literature, an inadequate graft tensioning is one of the most important reasons for complications during MPFL reconstruction such as stiffness, pain, cartilage degradation, arthrosis, patella fractures, etc. Therefore, authors’ balanced MPFL reconstruction technique might optimize the alignment of the medial soft tissue restraints with a correspondingly low incidence of complications. The authors technique is a practicable solution to achieve a feasible correction of the bony dysplasia combined with a balanced alignment of the medial soft tissue restraints. Thus, authors’ technique shows a reliable stabilization of the patellofemoral joint and a low incidence of complications. The results in common clinical outcome scorings as well as in terms of pain, return to sports and patient satisfaction are encouraging. To realize a simple combination of authors’ recently published techniques of a balanced MPFL reconstruction with sulcus deepening trocheloplasty, authors modified both techniques. Especially the described approach to the medial margin of the patella, the medial retinaculum and to the trochlear groove appears very feasible. On the one hand, the study presented here was a necessary first step in exploring authors’ modified method, which seems to provide reasonable advantages. On the other hand, this study gives an original insight to the patients‘ outcome. Besides the assessment of the commonly used scorings and the incidence of complications, this investigation gives a deeper insight to understand the outcome in terms of pain, cosmetic results, patient satisfaction and the return to sports. This might give us a better understanding of the patients‘ expectations and the role of physio- as well as an individual sports therapy. Because both trochlear dysplasia and an insufficiency of the medial soft tissue restraints are relevant for a symptomatic patellar instability, a method which addresses both patho-biomechanical conditions might be a good solution. A balanced medial soft tissue reconstruction might optimize the procedure. To combine authors’ technique of a balanced MPFL reconstruction to the bony alignment procedure, authors modified both the technique and especially the approach of the Bereiter trochleoplasty as well as authors’ MPFL reconstruction technique. Authors’ balanced MPFL reconstruction technique might help to get an adequate alignment of the medial soft tissue restraints. Besides the assessment of the commonly used outcome scorings, this study gives a better understanding of the outcome in terms of pain, cosmetic results, patient satisfaction and the return to sports. A new phenomenon author noticed in two patients was a persistent avoidance behaviour for typical stress situations during sports. Even if both patients reported that this was not instability- and/or pain-related, this behaviour interfered with their return to sports. Even if this phenomenon was seldom, these findings might highlight the need for an intensive physiotherapy as well as the need of an individual sports therapy. The modified technique shows encouraging results in terms of a sufficient stabilization of the patellofemoral joint, a low incidence of complications and a good outcome in terms of pain, cosmetic results and return to sports. The preliminary clinical results of authors’ technique demonstrate a good efficiency in relieving the symptoms and improving the function of the affected knee. Therefore, authors will continue this method in the future when indicated. In three patients, authors noticed a slight flexion deficit at the last follow-up. Considering this phenomenon, authors changed the postoperative treatment schedule to a protocol without any postoperative limitations of flexion. Furthermore, authors will try to ensure intensive postoperative motion exercises by using continuous passive motion devices, etc.

The comparatively good outcome presented in authors’ study should not be taken for granted. The need for an individual postoperative physio- and sports therapy is also outlined in this study. A larger clinical outcome study with longer follow-up periods is needed to investigate long-term outcome results of authors’ methods. This will assess the durability of the clinical results. Furthermore, data on the prevention of a secondary arthritis are of interest in this patient group, which normally shows high rates of osteoarthritis development. Besides further improvements of the surgical technique, the questions of how to optimize and individualize the postoperative physio- and sports therapy will be of interest. A larger long-term clinical outcome study on the clinical results and the prevention of secondary arthritis will be a useful method for the future research.