Dislocated patella

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

Luxating patella is a condition in which the patella, or kneecap, dislocates or moves out of its normal location. The luxation is usually medial, but can be lateral.

It can be caused by some form of blunt trauma, or may be a congenital defect. In congenital cases, it is usually bilateral.

Osteoarthritis can develop secondarily.

Symptoms can range from none to severe pain.

Diagnosis is made through palpation of the knee. The patient can do the patella tracking assessment by making a single leg squat and standing, or, lying on his or her back with knee extended from flexed position. A patella that slips medially on early flexion is called the J sign, and indicates imbalance between the VMO and lateral structures.^[1]

X-rays are necessary in some cases.

Supplements like glucosamine and NSAIDs can be used to keep the knee strong.

Grades III and IV, as well as most grade II cases, require surgery to correct, if the patient has difficulty walking. The surgery involves a sulcoplasty, a deepening of the trochlear sulcus that the patella sits in.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

There are four diagnostic grades of patellar luxation,^[1] each more severe than the previous:

• Grade I – the patella can be manually luxated but is reduced (returns to the normal position) when released;
• Grade II – the patella can be manually luxated or it can spontaneously luxate with flexion of the stifle joint. The patella remains luxated until it is manually reduced or when the animal extends the joint and derotates the tibia in the opposite direction of luxation;
• Grade III – the patella remains luxated most of the time but can be manually reduced with the stifle joint in extension. Flexion and extension of the stifle results in reluxation of the patella;
• Grade IV – the patella is permanently luxated and cannot be manually repositioned. There may be up to 90¼ of rotation of the proximal tibial plateau. The femoral trochlear groove is shallow or absent, and there is displacement of the quadriceps muscle group in the direction of luxation.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

Patellar dislocations occur by:

• A direct impact that knocks the patella out of joint
• A twisting motion of the knee
• A sudden lateral cut

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

It can be caused by some form of blunt trauma, or may be a congenital defect. In congenital cases, it is usually bilateral.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

In a journal by the Department of Orthopaedic Surgery, Brian Waterman collected incidence rate data in respect to sex, age and race in the U.S. Patellar dislocations have been estimated to occur among an at risk –population of 1,774,210,081 people per year.^[1]

The breakdown of the incidence rates found in AM J sports medicine journal, among the ages of 10–17 years old were found to be about 29 per 100,000 persons per year, while the adult population average for this type of injury ranged between 5.8 and 7.0 per 100,000 persons per year.^[2] The highest rates for this type of injury were found in the youngest age groups, while the rates declined with increasing ages.

Females are more susceptible to patellar dislocation.

Race is a significant factor for this injury, where Hispanics, African-Americans and Caucasians had slightly higher rates of patellar dislocation due to the types of athletic activity involved in basketball 18.2%, soccer 6.9% and football 6.9%, according to Brian Waterman.^[1]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

• Patellar dislocation occurs in sports that involve rotating the knee.
• Direct trauma to the knee can knock the patella out of joint.

• Twenty-four percent of patients whose patellas have dislocated have relatives who have experienced patellar dislocations.

• The manual of Structural Kinesiology by B.T Floyd states that people who have larger Q-angles tend to be more prone to having knee injuries such as dislocations, due to the central line of pull found in the quadriceps muscles that run from the anterior superior iliac spine to the center of the patella. The range of a normal Q angle for men ranges from <15 degrees and for females <20 degrees, putting females at a higher risk for this injury.^[1] Having an angle greater than 25 degrees between the patellar tendon and quadriceps muscle can predispose someone to patellar dislocation.^[2]

• In patella alta, the patella sits higher on the knee than normal.

• Normal function of the VMO muscle stabilizes the patella.
• If VMO function is decreased, it will result in instability of the patella.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Grace M. Gibson

Osteoarthritis can develop secondarily.

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