The objective of the present study is to compare the stability and alignment of the patella in patients with previous recurrent dislocation treated with medial patellar femoral ligament reconstruction with fascia lata allograft, 5 years after surgery by conventional CT and by CT under weight-bearing.

Conventional CT will be performed with the patient supine, the knee fully extended and with the muscles of the thigh and leg completely relaxed. On the same day, the patient will undergo a CT scan of the knee under weight-bearing with the Cone Beam Computed Tomography (CBCT) scanner system.

The CT images will be viewed through the PACS system by two independently expert radiologists who will measure the tilt, congruence angles and the TT-TG with the methodical notes described in the literature.

The use of a CT under weight-bearing allows for the first time to evaluate in a realistic way the stability and tracking of the patella, with the knee flexed and therefore with the activation of the quadriceps muscle.

Patellofemoral disorders make up about 20% of all knee pathologies and are often a source of diagnostic and therapeutic doubts for the orthopedic surgeon. The incidence of patella dislocation is estimated to be 5.8 cases per 100,000 people, with values that become five times higher in patients aged between 10 and 17 years. Conservative treatment is preferable in the first case of acute dislocation, however the recurrence of this episode occurs in a percentage equal to 15 - 44% in these subjects.

Many surgical interventions have been described to correct the various factors that predispose to patellofemoral instability, with variable success rates (72% with the Hauser technique and 93% with the Roux-Goldthwait technique, 73% with that of Fielding and 78% with that of Trillat after a long follow-up.

The medial patello-femoral ligament (MPFL) is one of the main stabilizers of the patella in its movement on the femur; numerous studies have recently shown that this ligament is damaged in almost all cases, thus recognizing the role of the structure that most limits the patella in its lateral dislocation to the MPFL. Following these studies, the reconstruction of the MPFL as an elective treatment for relapsing patella dislocation has been recently proposed with countless surgical techniques that use various types of grafts: autologous tendons (the first in 1990 by Suganuma et al), tendons from donor (allograft) and synthetic ligaments.

Our team recently developed a minimally invasive MPFL reconstruction technique using an allograft tendon of the fascia lata with the aim of stabilizing the patella and limiting the patellofemoral arthritic degeneration in the medium to long term.

The imaging evaluation of instability and correct alignment of the patella, both in the pre-operative and in the post-surgery, is normally performed using magnetic resonance imaging or conventional CT. The CT offers the advantage of being able to make a very accurate morphological evaluation and therefore to be able to calculate with great precision the degrees of the tilt angle of the patella, the subluxation and the distance between the tibial tuberosity and the center of the trochlear sulcus (TT-TG ). The big limit of this method is that the patient is acquired supine, with the knee extended and with the muscles of the leg relaxed while it is well known how the knee flexion and the contraction of the quadriceps muscle possible in load, significantly affect the tracking of the patella on the trochlea. In literature, numerous studies show that the acquisition of the lying patient removes these determining factors (Callaghan; Draper; Tanaka) by altering the assessment of instability and malalignment. In a recent study by Marzo et al. it has been shown that in subjects suffering from instability of the patella, the evaluation under weight-bearing and with knee flexed by CT (CBCT Carestream) shows lower tilt angles, congruence and TT-TG than the evaluation in load and therefore the evaluation under load of the patellar alignment is more adequate to evaluate with precision these parameters.

The objective of the present study is to compare the stability and alignment of the patella in patients with previous relapsing dislocation treated with reconstruction surgery by means of a medial patellar ligament with lata band allograft 5 years after surgery by conventional CT and by CT under weight-bearing.

The assessments are carried out in a group of 20 patients with previous recurrent patella dislocation who underwent reconstruction surgery between 2012 and 2013 via a medial patello-femoral ligament with fascia lata allograft at the Orthopedic and Traumatologic Clinic 2nd of the Rizzoli Orthopedic Institute. The patients will be contacted by phone and a follow-up visit will be scheduled which is normally carried out 5 years after the operation to evaluate the stability of the patella and therefore the patient's clinical condition. The patient will undergo a clinical examination and radiological control with conventional CT at the Rizzoli Orthopedic Institute. Conventional CT will be performed with the patient supine, the knee fully extended and with the muscles of the thigh and leg completely relaxed. On the same day, the patient will undergo a CT scan of the knee under weight-bearing with the Cone Beam Computed Tomography (CBCT) scanner system.The CT images will be viewed through the PACS system by two independently expert radiologists who will measure the tilt, congruence angles and the TT-TG with the methodical notes described in the literature.

The use of a CT under weight-bearing allows for the first time to evaluate in a realistic way the stability and tracking of the patella, with the knee flexed and therefore with the activation of the quadriceps muscle.

Inclusion Criteria:

• Who is able to provide informed consent and who agrees to sign the Informed Consent Form approved by the Ethics Committee (EC).
• Subject who underwent reconstruction of the MPFL using a minimally invasive technique using a fascia fascia allograft, performed at the Rizzoli Orthopedic Institute between 2012 and 2013 by the team of Prof. Zaffagnini.

Exclusion Criteria:

• patients with severe morphostructural deformities or other pathologies affecting the knee and lower limb
• patients with serious systemic vascular and neurological pathologies
• patients unable to perform a CT scan under weight-bearing
• State of pregnancy.
• Obese or with body mass index BMI> 30 kg / m2.
• Association of Trocleoplasty (intervention that produces degenerative changes of the joint in high percentage)