We report on a case of bilateral medial patellar dislocation, studied with hip/knee/ankle TC for evaluation of torsional defects and treated four times with distal derotative femoral osteotomy and cuneiform subtractive osteotomy of the lateral part of the femoral trochlea.

The patient, a woman 18 years of age at the beginning of treatment, presented with femoral neck anteversion of 30° dx and 25° sx referred to the plane passing posteriorly to the distal femoral condyles; the trochlear angle was 140° dx and 144° sx with medial inclination of trochlea due to medial hypoplasia. The patellae were facing medially, and clinically the patient had suffered medial dislocation of the patella several times.

The treatment lasted 4 years: the femoral derotation of 10° was executed by Orthofix monoassial external fixator; the lateral bone wedge removed from the trochlea was about 10 mm and we used reabsorbable nails for fixation.

Current values are 17° of femoral neck anteversion bilaterally, and the trochlear angle is 151° dx and 150° sx. The patient, now 26 years old, has not had any more dislocations and her knees have complete mobility with no pain.

The good result demonstrates the importance of femoral neck anteversion in the genesis of knee disorders. In this case the absence of specific abnormalities of the extensor mechanism (valgus knee, lateralisation of tibial apophyisis) probably caused the hypoplasia of the medial part of the trochlea and the resulting, rare medial patellar dislocation.

Based on the concepts of White and Panjabi of 1990 we have classified vertebral instabilities into congenital and acquired. The congenital instabilities are due to evident bone alterations that bring about mechanical instability, such as in spondylolisthesis and bone defects of formation and segmentation, or are caused by alteration of the elastic stability of the spinal column, such as in ligamental luxation of the Ehlers-Danlos syndrome or neuromuscular asthenia. Acquired vertebral instabilities include the extensive and much discussed issue of degenerative instabilities and secondary instabilities associated with rheumatoid arthritis, traumatic pathology, neoplastic, iatrogenic instabilities, etc.

The spine is a complex structure in elastic equilibrium between functional demand and the physiological resistance of the motor segments, that is the articulations, capsules and ligaments, and the muscles. Like Aulisa and Vinciguerra (1994) we are inclined to refer to stable and unstable equilibrium of the spine and to distinguish “mechanical” stability from “biological” stability. There are authoritative cases of evident macro-instability where the functional units, even though affected by serious mechanical alteration, are able to conserve a totally asymptomatic vertebral column in a state of elastic compensation for a long time.

We have classified our cases according to Christian Pfirrmann’s classification of lumbar intervertebral disc degeneration (2001), completing it with the three types of disc degeneration that Modic suggested in 1998. We present our case histories from 2001 to 2002 of macro-instabilities of lumbo-sacral spine treated with stabilisation, PLIF and fusion and of micro-instabilities treated with dynamic stabilisation in neutralisation without fusion.

In instabilities when one or more motor segments do not respond to permanent stress and the discs begin to change structurally and demonstrate phenomena of fissuring and dehydration, to the point of assured degeneration and collapse, we have developed a two-fold method of treatment:

MICRO-INSTABILITY: when the degenerative phenomena are still in progress and TAC, RMN and functional radiographs can identify an early phase, we propose dynamic stabilisation in neutralisation in order to restore the height of the disc and cancel the disc-radicular conflict, thus maintaining the capacity of movement of the functional unit.