Dr. Jonathan Mulford, Orthopaedic Surgeon, NSW Australia Total Knee Replacement, NSW Australia
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Knee Arthroscopy, NSW Australia

Knee Cap (Patella) Instability

Knee cap (patellofemoral) instability occurs when the knee cap does not slide centrally within the grove of the thigh bone (Trochlea). The knee cap can partially come out of the groove (subluxation) or completely come out of the groove (dislocation).

Our assessment of patients with patellofemoral instability has improved recently as our knowledge of patello-femoral joint anatomy and its stabilizing factors has increased.

Accurate assessment of the underlying abnormailities in the unstable joint enables formulation of appropriate management plans.


The delicate balance of bony and soft tissue stabilizers result in a joint that is prone to subluxation (half coming out of joint) and dislocation (fully coming out of joint).

Stabilisers of the Patellofemoral Joint – bones and soft tissues.

Bony Factors

  • Morphology of the trochlea and patella (the shape of the bones)
  • The knee cap (patella) usually sits within a trough (valley) on the front of the femur (trochlea)
  • The congruity (matching shapes) between the patella and the trochlea help with stability
  • In some patients the patella and trochlea do not form properly (patellofemoral dysplasia)
  • The trochlea is the predominant problem (trochlea dysplasia)
  • The normal trough of the trochlea is instead a flat surface or even dome shaped
Normal (Concave Trochlea) Trochlea Dysplasia (Flat trochlea)
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Bony alignment and Rotational Profile

In some patients the attachment of the quadriceps muscle to the shin bone (tibial tubercle) is abnormal.
This can be measured clinically and with a CT or MRI scan (called a TTTG distance).
Sometimes the bones can be twisted abnormally (rotational deformity).

Soft Tissue Constraints

If the trochlea is a shallow trough, soft tissue factors play a major role in stabilizing the patella. If you have lax ligaments (double jointed) there is increased risk of patella instability.

Static soft tissue constraints (ligaments and tendons).

The most important are:

  • Medial Patellofemoral Ligament (MPFL) – This can be torn with dislocations
  • Patella Tendon – If too long makes it difficult for the knee cap to stay in the trochlea

Patella Alta (Long Patella Tendon)
Knee Cap (Patella) Instability, NSW Australia

Dynamic soft tissue constraints

The thigh muscles help with stability of the patellofemoral joint.
The lower part of the inner thigh muscles (VMO) is commonly focused on in surgical soft tissue procedures and physiotherapy programs.

Clinical History

Facts about recurrent instability include

  • Female teenagers are the most common first time dislocators
  • After a first time dislocation there approximately 17% will have further problems
  • If previous subluxation or dislocation, 50% will have further episodes of instability
  • The younger the age at first dislocation, the higher the risk of subsequent dislocation
  • More severe initial dislocation = higher risk of recurrent dislocation
  • Further risk factors are a family history of patella instability and the risk factors for developmental dysplasia of the hip (first born girl, high birth weight, breech delivery, Caesarian)

Imaging of the Knee with Chronic Instability

These are the imaging techniques and measurements I use for reliability and simplicity and the benefit they provide in decision making.

  • X-rays These can show, Patella alta, Trochlea dysplasia
  • CT scans - CT scans have been widely used in the assessment of chronic instability. I mainly use CT scan to assess complex rotational alignment deformity
  • Magnetic Resonance Imaging (MRI) - MRI scans will reliably demonstrate
    • Bone abnormalities - Trochlea and patella dysplasia
    • Large cartilage injuries to the patellofemoral region
    • Patella tilt any abnormalities to the static constraints e.g. medial patellofemoral ligament rupture and patella alta
    • Bone bruising of a recent dislocation will be seen on these images
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Formulating a Surgical Management Plan

Surgery is only considered if non-operative treatment has failed and the recurrent nature of the disease has resulted in functional impairment.

Surgery may be directed toward either bone or soft tissue components.

I believe the surgical strategy should, wherever possible, aim to restore normal anatomy to the joint, rather than introduce new abnormalities. I will discuss what I believe will be the best option for you.

Bone Procedures

  • Rotational Oteotomy – Breaking the bone and fixing it in a new position to get the knee cap to track better
  • Trochleoplasty - For trochlear dysplasia where the cartilage of the trochlea is elevated and a trough in the bone is made for the patella to sit in
  • Tibial Tubercle Osteotomy – Moving the bony attachment of the patella

Soft Tissue procedure

  • Medial patellofemoral ligament reconstruction – Using the hamstrings tendon to reconstruct the MPFL
  • Lateral release – Releasing tight tissues on the outside of the patella
  • Medial imbrication – Moving the muscle acting on the patella
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Presence of Arthritic Change in the Patellofemoral Joint

In the presence of degenerative joint disease (cartilage damage), surgical treatment becomes difficult with a guarded outcome. If the articular lesion is localized to only one side of the joint then cartilage healing (micro fracture) or cartilage restorative (eg MACI or OATS) procedures can be used.

If degeneration is present on both joint surfaces, salvage surgery will be required.

Options include chondroplasty(shaving loose cartilage), lateral release (releasing tight tissue on the side of the knee cap) and tibial tubercle elevation (reseting the position of where the patella tendon attaches).

With failed surgery or advanced arthritis (wear and tear), Patellectomy (removing the knee cap), part knee replacement or total knee replacement are the only options.


The accurate assessment and management of the unstable patellofemoral joint depends on knowledge of the patello-femoral joint anatomy and stabilising factors. The surgical strategy should, wherever possible, aim to restore normal anatomy to the joint. This often involves a combination of surgical techniques.

Knee Replacement Surgery, NSW Australia
Sydney Orthopaedic Surgeon, NSW Australia

I am a orthopaedic surgeon that specialises in all aspects of Hip and Knee surgery...

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Dr Mulford was awarded a Diploma in Clinical Trials from the University of London in 2008.
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