Disorders of the Patellofemoral Joint Are a Common Cause of Knee Pain Throughout Adult Life

The anatomy of the knee joint

The knee joint is the hinge connecting the femur and tibia. This joint is extremely complex which allows not only for flexing and extending of the leg but also allows slight rotation of the tibia. The joint consists of the lower section of the femur, the upper section of the tibia, and the internal section of the kneecap, all of which are covered with a layer of cartilage. Between the femur and tibia, there are also “cushions” which serve as shock absorbers called meniscus (or menisci, plural form). The bones in the knee are connected by elastic structures called ligaments. There are cruciate and lateral ligaments which give the knee further stability without limiting the natural movement of the knee.

The articular capsule closes the entire joint and produces synovial fluid. The synovial fluid supplies the necessary nutrition to the surrounding cartilage and helps the joint lubrication providing movement without friction. The synovial fluid, together with cartilage and menisci, also has a shock absorbing function by intercepting strong forces which are then reverberated on joint during the life span of a person. The movement of the knee joint is possible due to the surrounding muscles.

Normal Knee Model and X-ray

Lateral view of the bones of knee joint Lateral X-ray view of the knee

/ [DSZ1]

What are the Compartments of the Knee

/ We divide the knee into three parts or compartments.
1. The inside of the knee between the femur and tibia (the medial part of the femoro-tibial joint)
2. The outside of the knee between the femur and tibia (the lateral part of the femoro-tibial joint)
3. Between the knee cap (patella) and the femur (the patello-femoral joint)
Each one of these compartments can be replaced separately with a partial (or unicompartmental) replacement or all three can be replaced with a total knee replacement.

Arthritis of the Patello-Femoral Joint

On occasions, arthritis can develop under the knee cap causing severe pain in the front of the knee only. The rest of the knee, that is the femoro-tibial joint is not affected by the degenerative process. This is thought to be due to an imbalance of the ligaments and tendons around the knee. It is called ELPS (Excessive Lateral Pressure Syndrome).The patella therefore tracts on the knee unevenly with a greater force on the outside of the knee (red arrow) than the inside (purple arrow). This results in excessive wear of the cartilage underlying the outside of the patella.It can result in disabling symptoms due to the advanced arthritis, at a relatively early age, as young as late thirties, early forties. /
In the above x-rays, there is bone on bone on the outside of the knee cap (red arrow) with preservation of the "joint space" of the knee joint proper, between the femur and tibia.
In the arthroscopic examination is demonstrated the appearance of the cartilage under the patella, which has worn down to the underlying bone. This is associated with significant inflammation within the knee.

incidence and aetiology

Disorders of the patellofemoral joint are a common cause of knee pain throughout adult life. For teenagers and those in early adult life the disorders present as non-specific anterior knee pain for which there can be a multitude of causes and can be a cause of troublesome disability. The exact incidence is unknown but with increasing age and progressive damage to the articular surfaces, established osteoarthritis of the patellofemoral joint develops. The condition is usually symptomatic causing moderate levels of pain and disability and particularly restrict the function of the knee. Paradoxically some patients can have well established osteoarthritic change present on radiographs without significant symptoms.

Typical isolated Patellofemoral arthritis

Increased awareness of these disorders has occurred as a result of more detailed investigation of the patellofemoral joint firstly with Merchant or Ficat’s tangential plain radiograph and secondly with the more widespread use of transverse images with CT and MR. Isolated patellofemoral disease exists in a proportion of 10% patients with osteoarthritis of the knee, and this process can remain localised for many years before progressing to the other compartments. The disease include isolated changes in either medial or lateral facets of the patellofemoral joint, or symetrical disease (Davies) and most are fundamentally related to the mechanical stability of the extensor mechanism.

This is a complex articulation transmitting force from the large extensor musculature over a narrow pulley to the tibial tubercle. Efficient function of this mechanism is essential for effective locomotion in the upright posture. Goodfellow and Hungerford described the contact bearing areas of the seven separate articulating facets of the patella and emphasised the complex overlap that occurs with the tibio femoral joint, unique in the animal kingdom. Minor degrees of incongruity with the trochlea will lead to considerable increase in the forces at the articular surfaces. Thus the patellofemoral joint is particularly vulnerable to minor degrees of incongruity and to the effects of injury. Mismatches in the articulation can result from a range of developmental malformations of either the patella or the trochlea. Alterations in the overall alignment of the limb both in varus, valgus or rotation can lead to incongruities and overload of the surfaces.

20° 45° 90° 135°

The stability of the patello-femoral joint is largely determined by the complex muscular and retinacular arrangements of the proximal extensor mechanism. Muscular imbalance of the quadriceps mechanism is a common cause of knee dysfunction particularly when there are underlying mal-alignments or incongruities. The factors determining muscular control are as yet ill understood and quadriceps dysfunction is sometimes difficult to correct. Once it has occurred, usually through the effects of injury, it often persists.These mechanical factors can lead on to articular cartilage damage with a progressive cycle of attrition, which is not easy to reverse.Studies of the pathology of advanced patellofemoral disease show that the lateral facet of the patella and trochlea are most commonly affected with 80% of cases demonstrating this pattern of lateral mal-alignment with subluxation. A small number of cases show medial facet and trochlea disease, (<10%). The precise aetiology of this condition is obscure but it would suggest that there may be an equivalent and opposite medial overload syndrome. The remaining cases demonstrate symmetrical patterns of wear affecting equally both the medial and lateral facets of the patella and trochlea groove.

Lateral facet OA Medial facet OA

Investigations

After a careful clinical history and examination, the investigations must include a weight bearing antero-posterior and lateral view of the tibiofemoral joint X-Rays films including the Rosenberg 30° PA view. Many cases of patellofemoral disease are missed because of the failure to take tangential views of the patellofemoral joint at 30° of flexion. This has been well described by Ficat and Merchant and is an essential radiograph to demonstrate the proximal part of the patellofemoral joint.

A/P and lateral weight bearing x rays and tangential view at 30° flexion.

Plain radiographs alone will usually demonstrate significant arthritic disease with joint space narrowing, osteophytes and subchondral sclerosis. The joint articulates through at least 130° and significant arthritic damage may be present in part of the arc of motion.

The use of dynamic CT scanning with views at 0°, 20° and 40° of flexion together with measurement of the height of the patella and tibial tubercle displacement will give a thorough investigation of the simple mechanics and structure of the patellofemoral joint.

QuadsrelaxedQuadscontracted

Dynamic stacked CT images of the P/F joint at 0° showing subluxation.

The use of NMR scanning will demonstrate in much more detail lesions of the articular surface on both the patella and the trochlea in its entire extent.

Dynamic MR studies are being developed to show the tracking of the patella throughout its early range of movement and these may eventually help to demonstrate specific malalignment syndromes

ProximalMiddleDistal

MR scans showing the upper middle and lower Patella facets in full extension.

The next step in the investigative sequence is the use of arthroscopy. This invasive technique should only be used when non-operative conservative measures have failed to control the symptoms. Useful information can be obtained and recorded about the state of the articular surfaces and the alignment of the patella. It must be remembered, however, that the magnification effect of arthroscopy can make articular lesions look several times larger than they are by direct vision. Treatment by chondrectomy can be performed at the same time to trim any loose unstable flaps of articular cartilage. Arthroscopically controlled extra or intra-articular lateral release can be performed but the place of this procedure remains controversial. In many cases the effects are short lived and it is probably only indicated in a small number of cases, which have early subluxation and tightness of the retinaculum. It is seldom effective on its own especially when there is any significant degree of lateral overload, subluxation or tilt.

Supero-lateral Portal to view patellofemoral joint.

The arthroscopic assessment should include a thorough assessment of the tibio femoral joint with a view of the patellofemoral joint, the inter-condylar notch and the lower trochlea surfaces from the inferolateral portal. An essential part of the assessment is to view the patella from the top of the supra-patella pouch usually by the supero-lateral portal. This portal can be easily identified by viewing the supra-patella pouch from below. The joint is then emptied of fluid and careful assessment of the position of the patella throughout the range of movement can be made. Static, or video images are taken of both the lateral gutter and the trochlea groove with the knee at 0°, 20°, 40° and 60° of flexion. Similar assessments are made after realignments procedures to ensure accurate correction of tracking.

Early Treatment

All patients should undergo a full course of non-operative conservative management prior to any invasive procedure. The use of anti-inflammatory analgesics in addition to intensive rehabilitation under physiotherapy control can solve many patients’ problems. The use of patella bracing and isokinetic rehabilitation provides a selection of options for patients with early disease.

Failure to improve constitutes the indications for further investigations and treatment. Radiographic assessment with dynamic CT scans or MR will provide useful information to assist with planning surgical treatment. Arthroscopic assessment will help to evaluate the alignment of the patellofemoral joint and give more detailed information about the state of the articular surfaces. It is important to recognise that the magnification effect of arthroscopy will make articular cartilage lesions appear several times bigger than they are in reality. Description of arthroscopic assessment and treatment has been fully described in the earlier section on investigations.

Arthroscopic chondrectomy can be beneficial to remove loose flaps of articular cartilage and tidy up areas of chondral irregularity. Although this will not promote healing of the chondral lesion, if followed by intensive rehabilitation, marked improvement in symptoms can occur in some cases. The addition of abrasion arthroplasty or microfracturing may be appropriate in more extensive and deeper lesions. If there is any degree of malalignment of the patella or impingement of chondral lesions as flexion occurs then distal tibial tubercle realignment procedures can be very helpful. Careful analysis of the precise characteristics of the malalignment and details of the position and extent of the chondral lesions is essential to plan the treatment. The French group in Lyon have pioneered realignment procedures for the treatment of lateral overload and subluxation using the Elmslie Trilat technique. This can be refined and performed under arthroscopic control. Preliminary arthroscopic assessment is performed as described above and an extra-articular mid lateral release of the lateral retinaculum performed maintaining the integrity of the lateral synovium.

This is performed under direct vision through a small lateral lazy S incision and the inferior lateral geniculate vessels can be secured. The Elmslie procedure is performed with an osteotomy of some 5cm to 7 cm leaving an intact tongue of bone at the distal end. Medial translation of the tibial tubercle can be performed up to 75% of the osteotomy surface and slight anteriorisation can be achieved by altering the angle of the osteotomy in the transverse plane. After initial displacement temporary fixation is performed to allow arthroscopic assessment of the patella tracking and stability. Further medial displacement can be performed if necessary though it is important not to exceed 75% of the osteotomy surface. The osteotomy is then secured with two fully threaded 4.0mm lagged cancellous screws. The heads countersink into the cortical bone of the tubercle and seldom cause problems. Petalling of bone on the lateral side will fill any gaps in the osteotomy and promote union.

Five years post operation of an Elmslie realignment for subluxation.

If there is persistent malalignment in the more severe degrees of lateral subluxation or persistent instability then a medial plication of the vastus medialis retinaculum can be performed through a small medial extra-articular incision. It is important not to over- tighten this repair for fear of producing medial overloadand even medial subluxation.

In cases with lesser degrees of lateral malalignment and subluxation Fulkerson’s osteotomy may be preferred. The angle of the osteotomy can be varied to create more anteriorisation or medialisation depending on the nature and position of the articular cartilage lesions of the patella and the degree of malalignment. Details of this procedure are described by Fulkerson (1) but considerable judgement is required to match the osteotomy with the chondral lesion.

In cases with pure lateral facet osteoarthritis, lateral facetectomy has been advocated by some authors. There is, however, no clear evidence of the quality or longevity of results from this procedure.

Patellectomy has traditionally been used as a salvage procedure for patellofemoral disorders. It has an uncertain outcome and many reports record good or excellent results in less than 50% of cases.Ref (2) Weakness of the extensor mechanism is an inevitable consequence and poor results can be expected if there is any degree of trochlea dysplasia or persistent malalignment of the extensor mechanism.

CT scan after patellectomy showing trochlear displasia and persistant subluxation

Treatment of Established Patellofemoral Disease with an arthroplasty.

Patellofemoral arthroplasty was first reported by McKeever in 1955 (3). Development of the procedure led to several designs being widely used in the 1970s. Reasonable short-term results have been reported using the Richards’ and Guepar (4 - 9) designs. The Lubinus prosthesis was designed in 1974 (Ref 10). The results have proved disappointing in the medium term with only 50% successful results at 7.5 years. Nevertheless nearly half the patients continued to experience excellent function and pain relief suggesting that improvements in design may eliminate some of the problems. Most series have reported difficulties with achieving satisfactory patella alignment and persistent subluxation leading on to wear has been a feature of many reports. The second major cause of failure is disease progression, which has been reported from 5% to 10% in most studies. Indeed Kooijman et al in a recent long-term review with a fifteen year follow-up, reported disease progression in over 20% of cases.

Indications

The indications for patellofemoral arthroplasty should be as strict as those for total knee replacement. There should be significant pain and disability. The use of functional scoring tools such as the reduced Womac score, the Oxford Knee Score and the Melbourne patella score can be helpful in assessing the severity of the condition.

Strict Indications for Arthroplasty

• Severe symptoms and signs
• Radiographically proven severe osteoarthritis of the PFJ
• No significant deformity, especially axis mal-alignment and fixed flexion
• "Normal" tibiofemoral joint

There is a large group of patients in their middle years who have got significant damage and dysfunction of the patello-femoral joint and would be much too young for a total knee replacement. Such patients should initially be treated with conservative surgical procedures including chondrectomy, articular cartilage grafting and realignment osteotomies. These treatments can often be successful for a number of years but progressive chondral damage is usually inevitable, leading on to early osteoarthritis. In such cases when there is severe pain and disability and all other surgical options have been exhausted an isolated patellofemoral arthroplasty may be considered as appropriate treatment.

Extended indications

• Failed realignment - Fulkerson/Elmslie
• Younger patient with early disease
• Dislocation & trochlea dysplasia
• Failed patellectomy
• Post - trauma (fracture or chondral damage)

Contraindications

The treatment of patellofemoral conditions is notoriously difficult. It is essential to make a specific diagnosis of the mechanical and pathological conditions and to plan the treatment appropriately.

There are certain conditions which are much more difficult to treat and they may constitute relative or absolute contra-indications for a patellofemoral arthroplasty.

1. Patella baja (infera)
   This condition is seldom successfully treated by arthroplasty because of the abnormal loading of the patellofemoral joint. This can be a developmental condition but may also be acquired following previous surgical treatment.
2. Patella alta
   This condition is often responsible for significant patellofemoral disease with either instability or arthritic damage. It may be appropriate to lower the patella with a tibial tubercle transfer prior to or at the same time as the patellofemoral arthroplasty. These cases may have significant proximal mal-alignment and great care must be taken with tracking and with the design of the proximal trochlea.
3. Algodystrophy – Regional pain syndrome
   These cases are notoriously difficult and great caution must be taken in considering an arthroplasty.
4. Arthrofibrosis
   This is often a secondary effect of an algodystrophy, or an abnormal response to previous surgery. It is generally unwise to attempt to treat such cases unless extremely good rehabilitation is available.

Axismal-alignment
If there is significant varus or valgus mal-alignment an isolated patellofemoral arthroplasty is probably