Voluntary femoro-tibial subluxations: a benign differential diagnosis in the snapping knee of a child.

PURPOSE: Voluntary femoro-tibial subluxation is a rare entity predominantly found in pretoddlers. It presents as a dynamic phenomenon with uni- or bilateral audible snapping of the knee, often in a context of fatigue or irritation at the end of the day. The aim of the study was to observe the evolution and recovery in these patients. METHODS: Ten children were included. Other causes of dislocating joints and pathologies with snapping of the lateral meniscus were excluded from this study. Six-week immobilisation with a splint at 70° of knee flexion was primarily recommended to all patients. RESULTS: The mean age at onset of symptoms was 10 months. Forty percent of the patients presented with bilateral symptoms. Clinically, four patients were hyperlax. In all patients, subluxations could be reproduced passively by rotating the foot externally and advancing the internal tibial plateau anteriorly. Three of the patients were treated with a dorsal splint and experienced significantly less or cessation of symptoms. Two patients underwent surgery, one for a concomitant anterior cruciate ligament (ACL) rupture and meniscus tear that worsened the symptoms, another for concomitant patella dislocation and a meniscus tear. One patient's parents refused treatment and four patients experienced less symptoms at the time of consultation and were not immobilized. Except for the two patients undergoing surgeries, no sequelae were observed. In general, the symptoms got less frequent when the patient began to walk. CONCLUSION: Voluntary femoro-tibial subluxation in children is a rare and benign condition that often resolves spontaneously without sequelae. The risk of meniscus tear should, however, be considered if subluxations do not cease. LEVEL OF EVIDENCE: IV.

Clinical management of adolescents and young adults suffering from sarcoma in the French Rhône-Alpes region: A prospective exhaustive cohort with 10 years follow up.

INTRODUCTION: Survival of adolescents and young adults (AYA) with sarcoma is lower than in younger patients. The objective of this study was to describe the regional healthcare circuits, the differences in the management between adult, paediatric and mixed units and to assess the prognostic impact of compliance with clinical practice guidelines (CPGs) on overall survival (OS) and on relapse free survival (RFS). MATERIALS AND METHODS: Retrospective analysis of the management and long term follow-up of all 13-25 year old patients with a sarcoma diagnosed in the Rhône-Alpes area between 2000 and 2005. RESULTS: 140 patients satisfied inclusion criteria and were selected. The majority of 13-25 year old patients were treated in paediatric units. Joint management resulted in a higher rate of discussion in multidisciplinary tumour board, inclusion in clinical trials, and fertility preservation. Non-compliance with guidelines was observed in 65% of cases. Overall compliance was not reported to correlate to survival. Compliance of radiotherapy with CPG's seemed associated with a better prognosis for OS (HR = 0.20, 95% CI = [0.10-0.40]; p < 0.0001) and RFS (HR = 0.18, 95% CI = [0.09-0.37; p < 0.0001) as well as compliance of surgery for OS (HR = 0.43, 95% CI = [0.23-0.81]; p = 0.01). Multivariate Cox regression analysis revealed other independent predictors of OS like age at diagnosis, stage and histological subtype. CONCLUSIONS: Management of AYA in joint units seems to improve the quality of care. Compliance of surgery and radiotherapy with CGP's seems to improve survival.

Vascularised fibula or induced membrane to treat congenital pseudarthrosis of the Tibia: A multicentre study of 18 patients with a mean 9.5-year follow-up.

BACKGROUND: The objective of this study was to compare outcomes of two surgical techniques used to treat congenital pseudarthrosis of the tibia (CPT), the induced membrane technique (IM) and the transfer of the contralateral vascularised fibula (VF). HYPOTHESIS: The IM technique produces similar outcomes to those of VF grafting in terms of healing and function, while being simpler and having a lower complication rate. MATERIAL AND METHOD: This retrospective multicentre study included 18 patients with a mean age of 2.8 years at surgery. Among them, 11 had neurofibromatosis type 1 (NF1). The IM technique was used in 10 patients and VF grafting in 8 patients. Mean follow-up was 9.5 years (range: 5-15 years). RESULTS: The two groups showed no significant differences for healing or the occurrence of complications such as limb length discrepancy and residual malalignment. Two patients required amputation, one in each group. The mean number of surgical procedures per patient was 4.7 in the IM group and 5 in the VF group. DISCUSSION: Outcomes are similar with the two techniques. Although VF grafting theoretically involves a single stage, the mean number of surgical procedures was not lower than after the IM technique. The IM technique was associated with lower risks of complications and residual donor site abnormalities. Regardless of the reconstruction technique, the quality of the initial bone resection and internal fixation, particularly regarding alignment, is of the utmost importance. LEVEL OF EVIDENCE: IV, comparative retrospective study.

Diagnostic value of ultrasonography in elbow trauma in children: Prospective study of 34 cases.

INTRODUCTION: Among the various elbow injuries in children that initially have normal radiographs, a certain number of occult fractures are only diagnosed correctly after the fact, during a follow-up visit. PURPOSE: This study evaluated the diagnostic contribution of ultrasonography in the treatment of acute elbow injuries in children and the strategic and economic impact of using this tool alongside radiography. MATERIALS AND METHODS: During this prospective study performed between January 1 and April 1 2014, elbow ultrasonography was performed within 6 days in all children under 15 years of age with a suspected occult fracture. The ultrasonography exam looked for lipohemarthrosis, the posterior fat pad sign and cortical disruption. If no fracture was visible on ultrasonography, a removable splint was given to the patient to relieve pain, and no radiological or clinical follow-up was scheduled. The patients were contacted again at least 15 days later to determine whether an undetected fracture was present. Lastly, we evaluated the cost of treatment with and without ultrasonography in the cases where no fracture was diagnosed. RESULTS: In 13 cases, ultrasonography revealed lipohemarthrosis and a fat pat sign, with cortical disruption also present in 11 of these cases. In two cases, the diagnosis was made based solely on the presence of lipohemarthrosis and a fat pat sign. There were seven lateral condyle fractures, two medial epicondyle fractures and two supracondylar fractures. Among the 21 patients with normal ultrasonography, no fracture was diagnosed later on. In patients without a fracture, using ultrasonography resulted in a cost savings of €29.10 per patient versus not using it. CONCLUSION: In our study, ultrasonography is a sensitive examination for the diagnosis of occult elbow fractures in children. When the radiography and ultrasonography are both normal, the possibility of fracture can be rule out definitively, which reduces the need for immobilization, follow-up and treatment costs. The findings of this preliminary study should be validated with a larger prospective study.

Bone mineral content and soft-tissue assessment in limb segments by dual-energy X-ray absorptiometry: optimal scan speed and pixel size.

The aim of this study was to define the best scanning parameters to assess bone mineral content (BMC) and bone mineral density (BMD) in specific dual-energy X-ray absorptiometry (DXA) applications, such as those encountered in orthopedics for the measurements of the peripheral skeleton. To simulate soft tissues and bones in a limb, we used simple phantoms made with plastic bottles (500 mL and 1500 mL) filled with tap water and cylinders of hydroxyapatite (HAP) powder set in a polymer at various concentrations (range, 0.150-1.500 g HAP/cm3). Data obtained from 204 measurements of these phantoms with different scanning speeds and pixel sizes showed that the best compromise for obtaining precise measurements in the shortest scan time was a scanning speed of 50 mm/s and a pixel size of 1.5 mm x 1.5 mm.

The value of early postoperative bone scan in slipped capital femoral epiphysis.

The purpose of this study was to evaluate the sensitivity and predictive value of early postoperative bone scan for detection of avascular necrosis (AVN) of the femoral head after surgical treatment of slipped capital femoral epiphysis. We reviewed records of 49 patients (64 hips) operated on between 1980 and 1997 with a mean follow-up of 3 years. Sixty-one out of 64 hips went through an early postoperative bone scan. The three hips that developed AVN showed significant loss of radionuclide uptake. There were neither false-positive or false-negative cases in this series. Early postoperative bone scan has an excellent sensitivity and predictive value for detection of AVN after surgical treatment of slipped capital femoral epiphysis.

[Pelvic obliquity and scoliosis in non-ambulatory patients with cerebral palsy: a descriptive study of 234 patients over 15 years of age]. / Bassin oblique et scoliose chez le patient infirme moteur cérébral non-marchant: étude descriptive de 234 patients de plus de 15 ans.

PURPOSE OF THE STUDY: Children with cerebral palsy who cannot walk have an oblique pelvis and scoliosis. There is a certain degree of controversy in the literature on the best way to manage this difficult situation. We present a descriptive analysis of a population of non-ambulatory adults with cerebral palsy in order to formulate hypotheses concerning the factors determining scoliosis. MATERIAL AND METHODS: This descriptive cross-sectional study was conducted in 234 patients aged over 15 years who had cerebral palsy and could not walk. Physical examination and an x-ray of the pelvis and spine in the reclining position were obtained for all patients. The following variables were recorded: luxation and subluxation of the hip, spontaneous deviation attitude, ability or not to turn over in bed, pelvic obliquity, history of bone surgery, defective hip abduction. The statistical analysis accounted for laterality and pelvis obliquity to the scoliosis convexity and the laterality of the hip excentration. RESULTS: Scoliosis was observed in 66.2% of the patients; it was more than 60 degrees in 34.5%. Two basic groups were distinguished: thoracolumbar scoliosis (41.6%) and lumbar scoliosis (41.6%). The prevalence of oblique pelvi was 59.9% with important difference by side: 31.6% right oblique and 68.4% left oblique pelvi. We were unable to find any relationship between the side of the pelvic obliquity and the side of the scoliosis convexity, the side of the hip excentration, or the deviation attitude, but the deviation attitude appeared to be a risk factor for pelvic obliquity, which itself was a risk factor for excentration, which was a risk factor for scoliosis. DISCUSSION: Scoliosis is an important problem in this population. Hip luxation is a direct risk factor for scoliosis, but the deviation attitude and pelvic obliquity are intermediary stages. The prevalence of oblique pelvi was greater on the left than the right. This finding should be confirmed in other series before hypotheses can be formulated concerning this difference.

[Dynamic anterior jerk of the shoulder. A new clinical test for shoulder instability. Preliminary study]. / Le ressaut dynamique antérieur de l'épaule. Un nouveau test clinique d'instabilité de l'épaule. Etude préliminaire.

PURPOSE OF STUDY: A new clinical test named "The shoulder anterior jerk test" is proposed to confirm the diagnosis of chronic shoulder anterior instability. MATERIAL: The test combines of a compression force and a translation force, applied along the arm between the humeral head and the glenoid cavity. In so doing, a subluxation of the humeral head is provoked and it is accompanied with a jerk recognised by the patient as his instability. METHODS: Three different populations of patients had been studied: Population A: 28 patients operated on for chronic anterior dislocation, uni or bilateral (32 shoulders), had been tested before the operation, without and under anesthesia. Population B: 100 patients without any problem at the shoulder, had been tested before and under anesthesia done for knee or hip surgery. Population C: 100 young sportive athletes with normal shoulder tested without anesthesia. RESULTS: Population A: The 28 patients suffering from shoulder instability had all a positive shoulder jerk test under anesthesia. Without anesthesia the test had been positive only in 10 cases (30 per cent). The jerk is potentially present for all the patients, but it is disturbed by apprehension. Population B: Among the 200 shoulders tested, 26 shoulders (17 patients) had a positive test under anesthesia (13 per cent). 5 had positive jerk test without anesthesia. Population C: 5 among the 200 shoulders tested had a positive jerk test (2.5 per cent) Under anesthesia the test has a sensitivity of 100 per cent, a specificity of 87 per cent, a positive predictive value of 55 per cent and a negative value of 100 per cent. Without anesthesia, the test has a sensitivity of 31 per cent, a specificity of 97.5 per cent, a positive predictive value of 66 per cent and a negative predictive value of 90 per cent. DISCUSSION: The instability which is shown by the jerk test is in relation with the anterior subluxation of the humeral head in front of the anterior edge of the glenoïd cavity. It reproducts, with a minimal amplitude, the clinical instability which is recognized by the patient. The test is always positive under anesthesia in case of chronic anterior instability, it may confirm pre-operative diagnosis just before the begining of the procedure and may orientate the choice of it. Apprehension is a major obstacle to the research of the jerk, but it is the same with the other classical clinical tests of the shoulder anterior instability. CONCLUSION: The anterior jerk test of the shoulder is thus a test which is able to prove the diagnosis of an anterior instability. The future will permit to confirm its efficiency and will confirm if the test may differenciate anterior and inferior instability with variation of the abduction, as it has been shown in this preliminary study.

[Avulsion fractures of the greater trochanter in children: two cases, review of the literature and proposition for a classification]. / Décollement épiphysaire du grand trochanter de l'enfant et proposition d'une classification. A propos de deux cas et revue de la littérature.

Avulsion fractures of the greater trochanter are very rare in children. We report two such cases which led to femoral head necrosis. Based on these two cases and an extensive review of the literature, we discuss the pathophysiology of this complication and propose a new classification system. Three types of lesions can be identified as a function of the mechanism causing fracture. Type 1 lesions are avulsion fractures of the greater trochanter secondary to acute contraction of the gluteus muscles. This contraction produces a vertical displacement of the greater trochanter. Femoral head necrosis has never been reported as a complication after this type of fracture mechanism. Type 2 avulsion fractures are associated with fracture of the femoral neck with a subsequent risk of femoral head necrosis. Type 3 associates hip dislocation with apophyseal avulsion with, according to the literature, an inevitable progression to head necrosis. The two cases reported look identical with those described by Linhart and Kawenblum illustrate type 3 avulsion fractures of the greater trochanter.

[Initial management of congenital varus equinus clubfoot by Ponseti's method]. / Prise en charge initiale du pied bot varus équin congénital selon la méthode de Ponseti.

The choice of first-line treatment for congenital varus equine clubfoot remains a controversial issue largely dependent on experience. In France, functional treatment predominates. In 1948, Ponseti proposed reducing the deformity with successive casts. Although cast treatment is a very old method, Ponseti's method is original because it is based on strict rules established from anatomic evidence. The goal is not to correct the apparent deformation, but on the contrary to impose a simultaneous supination and abduction of the foot. Once the calcaneopedal block has been derotated, percutaneous tenotomy of the Achilles tendon is performed. We relate our experience with this method and recall the precise technique used to make the casts. After the cast, derotation braces are worn at night but rehabilitation exercises are not required. We emphasize the quality of the clinical reduction achieved as well as the smaller number of patients who require surgery at walking age.

[The results after 10-16 years of the treatment of chronic anterior laxity of the knee using reconstruction of the anterior cruciate ligament with a patellar tendon graft combined with an external extra-articular reconstruction]. / Les résultats après 10 à 16 ans du traitement de la laxité chronique antérieure du genou par une reconstruction du ligament croisé antérieur avec une greffe de tendon rotulien associée à une plastie extra-articulaire externe. A propos de 138 cas.

PURPOSE OF THE STUDY: The aim of this prospective study was to evaluate the objective postoperative laxity and functional results with a minimum follow-up of 10 years (mean 11.7 +/- 2 years) in chronic anterior knee instability treated by ACL reconstruction associated to a lateral extra-articular plasty. MATERIAL AND METHODS: 138 patients of a mean age of 27.8 +/- 8.5 years had been operated. Delay between injury and operation was 4 +/- 4.8 years. The surgical "Mac Injones" procedure used a free autologous patellar tendon graft with a bone-to-bone fixation, supplemented by a lateral extra-articular plasty using a strip of quadriceps tendon as a direct prolongation of the graft of the patellar tendon and patella itself. A rehabilitation program aimed to an early recovery of a complete range of motion. Anterior laxity had been measured before and after operation using two instrumented methods, KT-1000 and stress-radiography (at 20 degrees of flexion with a 9 kg load applied at the distal part of the thigh) with measurements of the displacement in medial and lateral compartments. Tunnel positioning was appreciated radiologically. Function was evaluated using the International Knee Documentation Committee score (I.K.D.C.). RESULTS: Elongation of the reconstructed ligament occurred mainly during the first 6 month, but was independent from early full range of motion recovery. Laxity was stabilized after one year. The final laxity gain of the medial compartment was 62 per cent and for the lateral compartment 77 per cent. The pivot shift test was negative in 66 per cent, grade 2 in 4 per cent, grade 1 in 30 per cent. Functional results were excellent and good (A and B) in 60.4 per cent and 76.7 per cent returned to sports activity. 12 reconstructed ligaments reruptured. Arthritis was the cause of poor results in other cases (13.8 per cent). DISCUSSION: Lateral extra-articular plasty is unable to better control translation of the medial compartment than isolated anterior cruciate ligament reconstruction, but laxity of the lateral compartment was minimized and the pivot shift test also. Incorrect position of the anterior cruciate ligament was correlated with poor results. CONCLUSION: This documented study on laxity of the two compartments confirms the interest of each type of reconstruction, in particular extra-articular plasty when made with quadriceps tendon and so doing, preserving the iliotibial-band for the control of varus stability.

Patellar instability in children and adolescents.

Paediatric patellar instability encompasses many anatomic entities located along a continuum of knee extensor mechanism abnormalities. Major or minor clinical manifestations may occur at a variable age. In major forms with irreducible patellar dislocation or habitual patellar dislocation during knee flexion, shortness of the quadriceps is a consistent feature. A comprehensive aetiological work-up is in order, as syndromic conditions are common. Early surgical treatment is mandatory and should be performed by an experienced paediatric orthopaedic surgeon, as the procedure is technically challenging. Minor forms are more common; they are characterised by patellar dislocation or subluxation near terminal knee extension. The diagnosis may be difficult, particularly at the acute phase. Surgery is needed in patients with recurrent dislocation or functional impairments. The semiology of patellar instability has undergone considerable development in recent years, and a three-dimensional evaluation of patellar position can now be obtained using magnetic resonance imaging. Individually tailored surgical treatment "à la carte" remains a valid approach in 2013. However, new techniques for medial patello-femoral ligament reconstruction have modified the management strategies for adults and superseded many stabilisation procedures. Adapting these new techniques to paediatric patients and developing new procedures constitute major challenges.

Osteoid osteoma of the proximal femur: treatment by percutaneous bone resection and drilling (PBRD). A report of 44 cases.

INTRODUCTION: Osteoid osteoma is a benign osteogenic tumor that is mainly located in the lower limbs. According to Campanacci the proximal femur is involved in 25% of cases. We present a series of 44 cases of osteoid osteoma located in the neck of the femur or the lesser trochanter treated by the minimally invasive method, CT-guided percutaneous bone resection and drilling (PBRD). MATERIALS AND METHODS: This series included 44 patients, 20 girls and 24 boys, treated between 1987 and 2012. The average age at surgery was 12.7 years old (range 4-34). The diagnosis was based on the "association" of scintigraphy (hyperfixation) - CT scan (nidus located on the femoral neck or near the lesser trochanter). These patients underwent CT-guided PBRD under general anesthesia. Specific ancillary material was used to reach and remove the nidus and a cylinder of bone was sent to the pathologist for assessment. A lateral or anterior approach was used in all cases except one in which a posterior incision was made. Histological confirmation was obtained in 23 cases (the bone fragment was damaged in 21 cases). RESULTS: Forty-two patients were reviewed after a minimum follow-up of one year (12-56 months). Two patients were lost to follow-up. Results were evaluated clinically and on CT scan 1 year after surgery: there were 35 cures with complete and permanent pain relief. There were 5 failures and 1 case of recurrence requiring a second CT-guided PBRD procedure as well 2 complications involving femoral fracture (one associated with failure). DISCUSSION: The proximal femur is a common location of osteoid osteoma. Treatment requires careful preoperative planning to determine the surgical approach for safe removal. PBRD is a minimally invasive technique, allowing complete resection with suitable ancillary equipment. This method should be compared with thermoablation, which is a similar technique. CONCLUSION: CT-guided PBRD is a therapeutic option in case of osteoid osteoma of the proximal femur. LEVEL OF EVIDENCE: Level IV.

Induced-membrane femur reconstruction after resection of bone malignancies: three cases of massive graft resorption in children.

Bone reconstruction after surgical resection of bone malignancies in children remains a difficult challenge. Induced-membrane reconstruction as described by Masquelet et al. was originally reported in traumatic or septic bone defects and is now adapted to this field. We report here three cases of massive femoral graft resorption requiring surgical revision in two boys aged 3 and 6 years and a 9-year-old girl. Hypotheses include the long delay between the two stages, nature of the bone graft, high varus loads specific to this location, and lack of stability of the fixation. This technique has recently provided promising preliminary results when applied to the field of bone tumours. However, reconstruction of the femur seems to be specifically associated with a risk of graft resorption. Identification of the origin of this major complication is needed to amend the technique or its indications.

Osteoid osteoma of the acetabular fossa: five cases treated with percutaneous resection.

BACKGROUND: Osteoid osteoma is a painful benign osteogenic tumour for which the treatment objective is surgical resection of the nidus. The acetabular fossa is an uncommon site of involvement where surgical access can prove challenging. MATERIALS AND METHODS: We report a case-series composed of five patients with osteoid osteoma of the acetabular fossa treated with percutaneous bone resection and drilling under computed tomography guidance. RESULTS: All five patients had an uneventful postoperative course with immediate pain relief that was sustained over time. DISCUSSION: The outcomes achieved using our percutaneous technique compare favourably with those of other percutaneous methods, most notably regarding pain relief and patient tolerance of the procedure. CONCLUSION: Percutaneous bone resection and drilling under computed tomography guidance proved effective for the treatment of osteoid osteoma involving the acetabular fossa. LEVEL OF EVIDENCE: Level IV.

The introduction of a new MRI index to evaluate sagittal patellofemoral engagement.

INTRODUCTION: Patella alta is one of the primary factors of patellofemoral instability and its importance lies in the reduced engagement between patella and trochlea during the early degrees of flexion. The evaluation of patellar height is based on conventional x-rays, CT scan and, more recently, MRI. The objective of this multicentric prospective study is to describe a novel index to assess in the sagittal plane the functional engagement between patella and trochlea. MATERIALS AND METHODS: One hundred and thirty-five patients with objective patellar dislocation were prospectively enrolled between April 2010 and September 2011 and were compared with a second group of 45 controls. All patients underwent a standard MRI and a complete radiographic study. Sagittal engagement was measured as the ratio between the articular cartilage of the patella and the trochlear cartilage length measured on two different MRI slices. RESULTS: The mean Sagittal Patellofemoral Engagement (SPE) index was 0.43 ± 0.18 and ranged from 0.02 to 0.913 in the Objective Patellar Dislocation group versus 0.42 ± 0.11 range 0.22 to 0.55 in controls. In the Patellar Dislocation group the mean Caton-Deschamps index was 1.18 ± 0.21 (range 0.71 to 1.91). There were 58 patients with patella alta, in whom the mean SPE was 0.39 ± 0.18 (range 0.02 to 0.87). Sagittal engagement was significantly higher when compared with patients in the Patellar Dislocation group who had no patella alta (mean 0.46 ± 0.16, range 0.1-0.913). DISCUSSION: The present study introduces a new method to measure the SPE with the use of MRI. The evaluation of the functional engagement of the patella with the femoral trochlea in the sagittal plane can serve as a supplementary tool to the existing methods of evaluating patellar height, and may help to better identify the cases where inadequate engagement is recorded despite the absence of patella alta, so that the need for tibial tuberosity osteotomy may be re-assessed.

Axial MRI index of patellar engagement: a new method to assess patellar instability.

INTRODUCTION: The aim of this study was to define a new index to measure lateral patellar displacement (LPD) using nuclear magnetic resonance imaging (MRI), an axial index of engagement of the patella (AEI) obtained from two different axial MRI views then to validate its use in a prospective series of patients presenting an objective patellar instability (OPI). MATERIALS AND METHODS: One hundred and thirty-five patients with OPI and no history of surgery of the patella were included in a prospective study organized by the French Society of Arthroscopy performed between June 2010 and August 2012. All patients underwent axial and sagittal MRI. The AEI was obtained by projecting predefined patellar and trochlear landmarks (cartilaginous landmarks) on 2 different axial MRI views (one trochlear and one patellar). The results were compared with a series of controls (n=45). RESULTS: The preoperative AEI of the patella was 0.94 ± 0.09 for the control group and 0.84 ± 0.16 for OPI group (P=0.000016). The AEI could be obtained in 100% of the cases if it was measured on 2 MRI views while it could not be measured in 38.5% of the cases if the measurement was only obtained from one MRI view or whenever the widest part of the patella was not across from the femoral trochlea. The AEI did not significantly depend on dysplasia or the presence of a supratrochlear spur. The lowest AIE values were associated with trochlear dysplasia with a supratrochlear spur (P=0.0023) and a more prominent trochlea (P=0.0016). The AEI was correlated with patellar tilt (P<0.000001) and TT-TG on MRI (P<0.000001). DISCUSSION: AEI is a new index to measure LPD. It can be obtained in all cases because it is obtained from two different MRI views. The normal value is close to 1. It can be used to measure patellar instability on the axial plane in patients with OPI, especially in the most severe cases.