chapter 10: Patellar Dislocation
This abnormal relationship of the patella to the trochlea is due, in the majority of cases, to a patellar tendon that is abnormally long (Fig. 10.1) The relationship of patella alta to dislocation is the subject of some difference of opinion. Brattstrom (2) does not record this finding in his series. Ficat and Rizou (13) considered patella alta to be a significant factor in more than half of their cases. Insall et al (14) also thought that patella alta is an important etiologic factor. As in other abnormalities, it is clear that patella alta is not universally associated with dislocation, because many patellae that dislocate are normally situated. Neyret and the Lyon, France group have been working on proper indications for distalization of the patella in more severe cases of patella instability with patella alta.
The medial patellofemoral ligament(MPFL) reinforced by the vastus medialis would appear to be important in preventing dislocation., but the medial patella restraints are far more complicated than simply the MPFL. Additionally, there medial patellotibial, medial patellomeniscal ligaments, as well as a retinacular restaraint medially running from the medial patella to the medial collayeral ligament These may be weakened or torn. Koskinen and Kujala (15) have noted that the vastus medialis inserts more proximally in patients with patellar dislocation when compared with normal subjects. A primary dislocation, whether traumatic or due to constitutional factors, may so weaken the medial stabilizers that subsequent dislocations become likely. Recurrent dislocation has also been reported by several authors in syndromes of generalized ligamentous laxity including chondroosteodystrophy, Ehlers‑Danlos syndrome, and Marfan syndrome (10, 16) In these instances, the medial stabilizers are insufficient to resist the normal lateral tendency imposed by the Q angle.
Recurrent dislocation may occur after isolated vastus medialis paralysis, secondary to poliomyelitis. Other patients may manifest atrophy or hypoplasia of the vastus medialis. In the latter case, the muscle fibers attach only to the superior aspect of the patella as opposed to the superior and medial borders (11).
LATERAL RETINACULAR FACTORS
Abnormal fibrous insertion of the vastus lateralis, abnormal fascial bands from the vastus lateralis, and abnormally tight lateral retinaculum may play important roles in dislocation of the patella. Jeffries (17) reported three cases of abnormal attachment of fascia from the iliotibial tract to the superolateral border of the patella. However, in reviewing 76 cases, Heywood (8) did not see any cases in which this was the cause. All of this serves to underline the importance of many factors that contribute to the basic functional equilibrium of the extensor mechanism of the knee. Tight lateral retinaculum undoubtedly perpetuates lateral displacement of the patella and will increase the risk of dislocation.
Many authors have mentioned the tendency for recurrent dislocation to run in families. Bowker and Thompson (18) reported an incidence of 25% in their series of 48 patients, and Crosby and Insall (19) found 28% with positive family histories in their series. Down's syndrome also predisposes to patellar dislocation (20, 21), and there is a possible relationship between Turner's syndrome and recurrent patellar dislocation (22). Recurrent patellar dislocation also occurs in Kabuki make‑up syndrome (23).
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