TAKEHOME MESSAGE - The surgical technique of Untethering the Fat Pad

A provisional differential diagnosis of Anterior Knee Pain can be constructed as follows:

• AKP arising de novo (no previous surgery)
  • in a knee with an IPP-fat pad complex
  • in a knee without an IPP but with some other anomaly of the synovial layer
• AKP arising post-surgery on the knee
  • in a knee with an intact IPP-fat pad complex
  • in a knee in which the IPP has been resected

Our current understanding about adolescent AKP is that:

• there is no known cause of this problem but it arises

  • insidiously at the time of the onset of the growth spurt, when the 2 growth plates in either side of the knee, which have the largest velocity of growth in the body, are growing in opposite directions, essentially compressing the contents of the anterior compartment, the IPP and fat pad, and inducing the necessity for rapid growth of all of the other soft tissues to counter-balance the forces arising from growth of the femur and tibia.
  • in association with overuse which may induce micro-damage of structures, and activitation of cytokines which activate the noxious nervous network in the fat pad and other structures
  • in association with injury with associated activation of cytokines and associated noxious nerves.

• regardless of the cause, once adolescent AKP is established, in each child it becomes a complex problem with a short timeline of months before patterns of chronic pain become established.

In those patients with AKP where the symptoms appear to be related to the presence of an IPP-fat pad complex, we have outlined the technique and scientific basis for a simple, safe operation that can be performed early (we suggest 3-6 months) if non-operative management fails. The principles of this surgery are summarized here:

• First principle: Untether the fat pad, eliminating any central restraint; release and/or resect the IPP and any other attached structure that interferes with the freely reversible deformation and stretch of the fat pad (anomalous bands).
• Second principle: Restore, as much as possible, the anatomic contours of the borders of the anterior compartment, eliminating interference with the fat pad as it deforms with knee motion. This implies removal of local pathology, which is uncommon, such as the following:
  • Cyclops lesion attached at the base of the reconstructed ACL
  • Arthrofibrotic scarring
  • Attached tissues which may be osseous, cartilaginous, or both (osteochondromatous) or other soft-tissue lesions (cysts, focal PVNS, or any soft-tissue tumor)
• Third principle: Because the fat pad is a structure of multiple functions, disturb it as little as possible.
• Fourth principle: Because the IPP-fat pad complex is a potential pain generator, consider untethering the fat pad every time you perform arthroscopy on a knee for undiagnosed pain.

At the bottom of this page is a link to our procedure as an open-access publication.

Caveat

Because AKP may occur in patients without an IPP substantial questions remain. To seek answers, it is clear that prospective studies need to be performed.

We have evaluated such patients on rare occasions. These patients had no IPP but had all the “classical” features of AKP. We have very limited experience with such patients. Two memorable cases had anomalies (congenital malformations) of the synovial layer, one a medial shelf, a broad band of dense connective tissue which spanned the inner wall of the synovial layer from above (superomedial) to the central body below, which appeared as a nodular structure in the midline of the fat pad. In the other case the anomaly was another thick band of dense connective tissue in the superolateral recess spanning from anterior to posterior. In both instances the patients were adolescent athletes. In each, the lesion was judiciously resected, and the AKP resolved. Our conjecture is that the innervated synovial layer was the source of the pain.

Scott Dye (ref. 1) had demonstrated that this synovial layer throughout the knee was moderately sensitive to mechanical pressure, and severely sensitive in the fat pad and surrounding structures. Our model for AKP that arises de novo, in the absence of previous surgery, is that the pain arises from perturbed noxious nerves contained within the fat pad and immediately adjacent structures. Most of the time the perturbations arise from the common factor that is the physiology of the IPP-fat pad complex. In situations where there is no IPP, our conjecture is that any dense connective tissue structure that substantially spans the synovial space and perturbing the fat pad can mimic the AKP associated with the IPP-fat pad complex.

After our paper was published, orthopaedic surgeons, who specialize in the management of limb alignment, and patellar instability, indicated to me that they often see AKP in multiple-operated knees in which the IPP has been resected. In such patients the physiology that is normally associated with the AKP is gone. Given the clear association between the release of the IPP-fat pad complex and relief of AKP, one could ask: Do such patients manifest all of the elements of the symptom complex of AKP or is their knee pain different in some way? Our experience is limited in the multiple-operated knee. We look forward, over time, to have an increased opportunity to evaluate such patients.

One of the paradoxes of AKP is the fact that patients who have malalignment will often have AKP in just one knee. Clearly, the mechanical factors of limb alignment (axial, rotational, patellar tilt) and patellar instability play an important role in the generation of osteoarthritis and in the generation of knee pain. The pain of osteoarthritis, however, lacks the features of AKP (see the discussion on pain above). The relationship between AKP, malalignment and osteoarthritis is a field of active research study at this time.