There are many reasons for patella to dislocate; and one must understand some basic facts before going in details of the patella dislocation.

Basic Understanding

Patella is a small bone situated in the front of the knee and is usually called the ‘knee cap’. Though small, it has a very complex mechanism to play for the perfect functioning of the knee joint. The 'knee cap' slides up and down with the knee movements in a groove called ‘Trochlea”. Trochlea or ‘the groove’ is a “V” shaped structure that keeps patella in its slot thru out the knee movement. In addition to the groove, patella is aslo kept in the ‘V’ slot or the groove by two major ligaments on either side of it. These ligaments work as a string attached on either side of the patella while patella is moving up and down in the groove or the ‘V’ or the trochlea. 

1. MPFL tear

If one of these strings (ligaments) gets torn, then patella dislocates in the direction opposite of the torn ligament. By design of the knee and lower limb, patella has a tendency to dislocate towards outside of the knee joint; almost always. Dislocation of the patella towards inside of the knee (or towards opposite knee) is a very rare occurrence. This means that almost always, the ligament (the string) on the inner side of the patella gets a tear. This inner string is known as Medial Patellofemoral Ligament (MPFL).

The disloction of patella may be a single episode or may proceed to recuurent episodes. 

2. Shallow Trochlea

If the trochlea or the groove or the “V” is shallow then also; patella will have tendency to slide out of the groove resulting in patella dislocation.

3. Shallow Trochlea leading to MPFL stretch out

Due to shallow trochlea, the MPFL or the medial sling strteches out gradually over a period of time. This is the most common cause of the patella dislocation.

4. Other causes

Apart from these two important causes (the torn MPFL or the abnormal trochlea or the groove); there can be several other causes that can lead to patella dislocation. For example, some congenital or developmental abnormality at hip joint, abnormal angle between thigh bone and the leg bone, abnormal development of hip-knee-ankle alignment during growth period, generalized joint laxity etc. 

Dr Deepak Goyal

Consultant Cartilage, Arthroscopy and Sports Knee Surgeon.

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Dr Goyal has shared his wide experience related to patella dislocation and patella disorders at many international and national platforms. Click below links to know more.

#. Emirates Orthopedic Society

#. Arthropreserve, Chennai, 2015: ‘The superficial quad technique’ for single stage management of patella instability and patellar osteochondral fracture.

#. Asian Cartilage Repair Society, Seoul, Korea, 2015: ‘The superficial quad technique’ for concomitant osteochondral fracture of patella an patella instability.

#. Tehran Live Surgery Arthroscopy Course, Tehran, Iran, 2014: Live surgery demonstration of ‘the superficial quad technique’ for MPFL reconstruction.

#. Tripler Army Hospital, Northern Pacific Command, Honolul, Hawaii, USA, 2014: MPFL reconstruction for patella dislocation using ‘the superficial quad technique’.

#. Rush Hospital, Chicago, USA: MPFL reconstruction- an innovative technique called ‘the superficial quad technique’.

#. Cleveland Clinics, Cleveland, USA: ‘The superficial quad technique’ to treat patella instability.

#. Iowa University Hospital, Iowa, USA: MPFL reconstruction using ‘the superficial quad technique’.

#. Tria Hospitals, Minnesota, USA: MPFL reconstruction using ‘the superficial quad technique’.

#. European Society of Sports Traumatology, Knee Surgery and Arthroscopy, Netherlands, 2014: MPFL reconstruction- technique using quad tendons.

#. Rajkot Orthopedic Society, Rajkot, India, 2014: Clinical approach to patient with painful patella.

#. Rajkot Orthopedic Society, Rajkot, India, 2014: Management of unstable patella using ‘the superficial quad technique’.

#. Rajkot Orthopedic Society, Rajkot, India, 2014: Concomitant management of patella osteochondral fracture and patella instability.

#. Rajkot Orthopedic Society, Rajkot, India, 2014: Management of cartilage defects of patella.

#. Indian Arthroscopy Society, Hyderabad, India, 2014: Trochleoplasty for patella instability.

#. Japanese Orthopedic Association, Hiroshima, Japan, 2013: ‘The superficial quad technique’ for MPFL reconstruction.

#. Indian Arthroscopy Society, Mumbai, India, 2013: MPFL reconstruction- ‘the superficial quad technique’.

#. Indian Arthroscopy Society, Mumbai, India, 2013: Understanding various technique of MPFL reconstruction.