The patellofemoral prosthesis, also known as patellofemoral replacement or PFJ (Patellofemoral Joint) prosthesis, is a special form of partial joint replacement used in isolated arthritis of the patellofemoral joint. Despite its theoretical advantages, such as sparing the unaffected joint sections, it has a poor reputation in orthopaedics. But why is that? This article examines the reasons for the scepticism towards this prosthesis in detail, starting with the diagnosis of patellofemoral arthritis and moving on to the challenges and improvements of the patellofemoral prosthesis.

Anatomy and function of the patellofemoral joint

The knee joint consists of three main joint sections: the medial and lateral femorotibial joint and the patellofemoral joint. The latter involves the interaction between the patella and the femur. The main function of the patella is to improve the leverage of the quadriceps muscle and optimize the transmission of force to the knee joint. However, due to the stress in everyday life and especially during sports activities, this area is prone to osteoarthritis.

Retropatellar osteoarthritis: definition and causes

Retropatellar arthrosis refers to the wear and tear of the articular cartilage between the patella and the femur. The most common causes include:

• Misalignments such as a patellar dysplasia or a patellar dislocation
• Cartilage damage through trauma or overuse
• Incorrect loading due to muscular imbalances
• Postoperative changes after previous knee surgeries

Symptoms of Retropatellar Osteoarthritis

Patients with retropatellar arthrosis typically report:

• Pain during stress, especially when climbing stairs
• Start-up pain after prolonged sitting
• Grinding noises (crepitations) during movements
• Swelling and limited mobility

Diagnosis of retropatellar osteoarthritis

The diagnosis of retropatellar arthrosis is based on a combination of clinical examination and imaging procedures. These include:

• Clinical tests, e.g. the patellar compression test
• X-ray images, especially in special views (e.g., Sunrise view)
• MRI examinations to assess cartilage damage

Conservative Treatment Options

Before surgical treatment, conservative therapy is usually recommended, consisting of:

• Physiotherapy to strengthen the surrounding musculature
• Pain medication, e.g. NSAIDs
• Injections with hyaluronic acid or PRP
• Orthopedic aids, e.g., patellar braces

Surgical treatment options

If conservative therapy is not sufficient, various surgical options are available:

• Arthroscopic cartilage smoothing
• Realignment osteotomy to correct misalignments
• Patellofemoral prosthesis as a selective joint replacement

The patellofemoral prosthesis: indications and procedures

The indication for a Patellofemoral prosthesis (PFJ prosthesis) is of crucial importance, as it is suitable only for a very limited patient group. An inaccurate or overly generous indication can lead to unsatisfactory results, renewed pain, and ultimately a necessary revision surgery.

Why is the indication so crucial?

The patellofemoral prosthesis merely replaces the bearing surface between the kneecap (Patella) and the femur. It is therefore a partial prosthesis, which is exclusively for patients with an isolated retropatellar arthritis (arthritis of the kneecap joint) is applicable. This means:

✅ Suitable indication:

• Primary retropatellar arthritis: The arthritis is exclusively confined to the patellofemoral joint, without signs of wear in the other joint compartments (medial or lateral compartment).
• Post-traumatic Retropatellar Osteoarthritis: After a patellar fracture or other injuries, selective osteoarthritis can develop in the patellofemoral joint.
• Dysplasia-related retropatellar arthritis: In patients with a congenital malformation of the kneecap or the associated trochlear groove in the femur, an isolated arthritis can develop prematurely.

❌ Contraindications – When is a PFJ prosthesis unsuitable?

• Generalized knee osteoarthritis: If other joint parts are already affected, a Total endoprosthesis (knee-TEP) is usually the better choice.
• Ligament weakness or Instabilities: An isolated PFJ prosthesis does not stabilize the knee, therefore patients with ligament instabilities or axis deformities are unsuitable.
• Malalignment of the patella (Maltracking): An insufficiently aligned patella leads to unfavorable pressure conditions and poorer prosthesis function.

Conclusion: Only very few patients are suitable candidates

Due to the strict indication criteria, a PFJ prosthesis is only suitable for a very small proportion of patients. Many patients with pain in the front of the knee suffer either from general knee osteoarthritis or biomechanical problems, which a PFJ prosthesis cannot solve. generellen Kniearthrose oder biomechanischen Problemen, die eine PFJ-Prothese nicht lösen kann.

Therefore, a thorough diagnostic assessment by a specialist is essential before a patellofemoral prosthesis is considered. Only when the indication is exact fits, this partial prosthesis can provide a good long‑term solution.

Why does the patellofemoral prosthesis have a bad reputation?

Although the idea of an isolated joint replacement is promising, there are various factors that have contributed to the skepticism towards this prosthesis:

• High revision rate: Studies show that many patients later require a total knee replacement.
• Biomechanical problems: The prosthesis must be optimally aligned to avoid complications such as malalignment.
• High patient expectations: Patients often expect complete pain freedom after the surgery, which cannot always be guaranteed.

Patient selection and preoperative planning

The correct selection of patients is crucial for the success of a patellofemoral prosthesis. Criteria include:

• Evidence of isolated retropatellar arthrosis
• Absent or minimal degenerative changes in other knee compartments
• Stable ligaments and no advanced axial malalignment

Rehabilitation after patellofemoral arthroplasty

Postoperative aftercare plays a significant role in therapeutic success. The most important measures include:

• Early mobilisation, to avoid muscle atrophy
• Targeted strength training to stabilize the knee
• Coordination training for improved mobility

Conclusion

The patellofemoral joint replacement (PFJ) can be a good solution for selected patients, but it suffers from a poor reputation due to previous negative experiences. Modern developments and improved patient selection could help increase the acceptance of this prosthesis type in the future.