Patella infera described by Caton et al.The measurement is made in 1982 is an accompanying symptom in certain knee affections secondary to the abnormal situation of the patella. The measurement is made on the X-ray with sagittal view after measuring the patellar height, using the original technique described by the author, when the ratio between the articular surface of the patella and the distance form the patellar tip to the tibial tubercle. Indications of surgery may be when this ration is inferior or equals 0.6. The origin of the patella infera can be mechanical or inflammatory. The operative technique addresses the etiology. In current practice, the patellar height and the patellar tendon length may be evaluated using a sagittal section MRI. In the authors' experience, when the Caton ratio is lower or equals 0.6 and when the length of the patellar tendon is over 25 mm, the indication of surgery includes proximal transfer of the tibial tubercle. If the length of the patellar tendon is less than 25 mm, it is often necessary to perform a patellar tendon lengthening (PTL). This type of surgery is contraindicated in the authors' experience in depressive or pusillanimous subjects. The two surgical techniques are described. Both techniques use an anterior and medial approach. The proximal transfer of the tibial tubercle (PTT) includes medial and lateral retinaculum release. The tibial tubercle is detached and transferred upwards according to the pre-operative planning generally 1 or 2 cm and is fixed with 2 screws. PTL includes a medial and lateral retinaculum release often with the fat pad. The patellar tendon is dissociated in the middle over its whole length, and the medial pad is detached of the tibial tubercle and the lateral of the patella. After lengthening, the edges of the tendon are sutured, and this suturing reinforced. Alternative procedures may be used when PTT or PTL are not possible, using transplantation with an allograft of the extensor system or a plasty with hamstring muscles.
