The patellofemoral joint: from dysplasia to dislocation

Abstract

Patellofemoral dysplasia is a major predisposing factor for instability of the patellofemoral joint. However, there is no consensus as to whether patellofemoral dysplasia is genetic in origin, caused by imbalanced forces producing maltracking and remodelling of the trochlea during infancy and growth, or due to other unknown and unexplored factors.The biomechanical effects of patellofemoral dysplasia on patellar stability and on surgical procedures have not been fully investigated. Also, different anatomical and demographic risk factors have been suggested, in an attempt to identify the recurrent dislocators. Therefore, a comprehensive evaluation of all the radiographic, MRI and CT parameters can help the clinician to assess patients with primary and recurrent patellar dislocation and guide management.Patellofemoral dysplasia still represents an extremely challenging condition to manage. Its controversial aetiology and its complex biomechanical behaviour continue to pose more questions than answers to the research community, which reflects the lack of universally accepted guidelines for the correct treatment. However, due to the complexity of this condition, an extremely personalised approach should be reserved for each patient, in considering and addressing the anatomical abnormalities responsible for the symptoms. Cite this article: EFORT Open Rev 2017;2. DOI: 10.1302/2058-5241.2.160081. Originally published online at www.efortopenreviews.org.

Keywords: dislocation; dysplasia; instability; knee; luxation; patella.

Fig. 1

Radiographic features of patellofemoral dysplasia: a) normal deep trochlea; b) ‘crossing sign’ with a flat trochlea; c) ‘double contour sign’ with a convex trochlea.

Fig. 2

Classification of patellofemoral dysplasia according to Dejour: a) type A with ‘crossing sign’; b) type B with ‘crossing sign’ and ‘supratrochlear spur’; c) type C with ‘crossing sign’ and ‘double contour sign’; d) type D with ‘crossing sign’, ‘double contour sign’ and ‘supratrochlear spur’.

Fig. 3

CT scan evaluation of patellofemoral dysplasia: a 21-year-old female patient with bilateral patellar instability due to type D dysplasia.

Fig. 4

Tibial tuberosity – trochlear groove distance measurement: the deepest point of the trochlear groove (TG) is obtained in the axial cut and superimposed in a second axial cut where the anterior tibial tuberosity (TT) is marked. The distance between these two points is measured.

Fig. 5

Measurement of lower limb axial rotational alignment: a) femoral rotational alignment is measured calculating the angle between the line running through the centre of the femoral head (FH) and the centre of the femoral neck (FN) and the posterior bi-condylar line (PC); b) tibial rotational alignment is measured calculating the angle between the tangent line to the posterior condyles (PC) and the axis of the ankle joint (AA).

Fig. 6

Evaluation of patellar height: a) the Insall-Salvati ratio is measured on a true lateral radiograph with the knee at 30° of flexion, as the ratio between patellar tendon length (LT) and the length of the patella (LP); b) similarly, the Caton-Deshamps ratio is measured as the ratio between the distance from the lower pole of the patella to the upper limit of the tibia (AT) and the length of the patellar joint surface (AP).

Fig. 7

Sagittal patellofemoral engagement (SPE) index measurement: a) the SPE index is measured using a sagittal MRI view, with the knee in full extension. The first cut is determined where the patella shows the longest articular cartilage. On this image, a patellar length (PL) line is drawn, that measures the entire length of the patellar articular cartilage. The second cut is selected where the femoral trochlear cartilage extends most proximally. On this slice, the PL line copied from the first cut, is inserted. A second line is then drawn parallel to the PL, which starts from the most proximal articular trochlear cartilage and finishes at the distal end of the PL, providing the trochlear length (TL) line. The SPE is calculated as the ratio between TL and PL. b) Axial engagement index (AEI) measurement: the AEI is measured using an axial MRI view, with the knee in full extension. The first cut is chosen where the lateral border of the trochlea is largest; the posterior bi-condylar line (PC) is drawn. A perpendicular line (T) to the PC line is drawn passing from the most lateral point of lateral trochlear border. A second cut is chosen where the patella is widest; after transferring the BC in this slice, a perpendicular line (M) is drawn passing the most medial part of the patellar cartilage. The distance (TL) between the line T and M is measured. The distance (PL) between most lateral point of the patellar cartilage (L) is measured. The AEI of the patella is equal to LP/LT.

Fig. 8

Patellofemoral biomechanics during knee flexion: a) in the sagittal plane the quadriceps muscle tension and the patellar tendon tension combine to produce a resultant force vector directed posteriorly on the patella; this force vector is minimal in the first degrees of flexion (force 30°), while increasing progressively at higher degrees of flexion (force 90°), stabilising the patella into the trochlear groove. b) In the coronal plane, the angle between the force of the quadriceps muscle and the line of action of the patellar tendon is known as ‘Q-angle’. This tends to increase from knee flexion, where the tibia internally rotates ‘medialising’ the tibial tubercle (Q-angle at 90°), to full extension, where the tibia externally rotates ‘lateralising’ the tibial tubercle (Q-angle at 0°). A higher ‘Q-angle’ represents a disadvantageous condition for patellar tracking. TT, tibial tuberosity.

Fig. 9

Surgical steps of medial patellotibial ligament (MPTL) reconstruction: a) the patellar tendon is incised to isolate its medial third, which should be detached with a tibial tuberosity bone plug; b) the bone plug is fixed on the medial tibia with a metal screw, anteriorly to the medial collateral ligament (MCL).

Fig. 10

Medial patellofemoral ligament (MPFL) anatomy and reconstruction. Anatomical specimen showing shape and insertions of the MPFL: a) schematic drawing of MPFL reconstruction with fascia lata allograft and b) fixation with anchors and metal screw.