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:

• Malalignment such as patellar dysplasia or patellar luxation
• Cartilage damage due to trauma or overload
• 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-rays, especially in special planes (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 straps

Surgical treatment options

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

• Arthroscopic cartilage smoothing
• Corrective osteotomy to correct malalignments
• Patellofemoral prosthesis as selective joint replacement

The patellofemoral prosthesis: indications and procedures

The indication for a patellofemoral prosthesis (PFJ prosthesis) is of crucial importance, as it is only suitable 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 only replaces the bearing between the kneecap (patella) and thigh bone (femur). It is therefore a partial prosthesis, which is exclusively suitable for patients with isolated patellofemoral arthritis (arthritis of the patellofemoral joint). This means:

✅ Suitable indication:

• Primary Retropatellar Osteoarthritis: The osteoarthritis is limited to the patellofemoral joint, without any 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 osteoarthritis: In patients with a congenital malformation of the patella or the associated femoral trochlea, isolated osteoarthritis can develop prematurely.

❌ Contraindications – When is a PFJ prosthesis unsuitable?

• Generalized knee arthrosis: If other joint parts are already affected, a total knee arthroplasty (TKA) is usually the better choice.
• Ligament weakness or instabilities: An isolated PFJ prosthesis does not stabilize the knee, so patients with ligament instabilities or malalignments are unsuitable.
• Patellar maltracking: An inadequately aligned patella leads to unfavorable pressure conditions and poor function of the prosthesis.

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 anterior knee area suffer from either general knee osteoarthritis or biomechanical problems that a PFJ prosthesis cannot solve.

Therefore, a thorough diagnostic evaluation by a specialist is essential before a patellofemoral prosthesis is considered. Only if the indication exactly fits, can this partial prosthesis be 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: Often, patients expect complete pain relief after 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 mobilizationto prevent 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.