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, 7 (5), e575-e588
eCollection

Arthroscopic Untethering of the Fat Pad of the Knee: Release or Resection of the Infrapatellar Plica (Ligamentum Mucosum) and Related Structures for Anterior Knee Pain

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Arthroscopic Untethering of the Fat Pad of the Knee: Release or Resection of the Infrapatellar Plica (Ligamentum Mucosum) and Related Structures for Anterior Knee Pain

Thomas Victor Smallman et al. Arthrosc Tech.

Abstract

Anterior knee pain (AKP), a multifactorial symptom complex, can be successfully treated surgically. A specific diagnosis often cannot be made, but the pain is linked to an unrecognized common factor in most patients: the mechanical behavior of the non-isometric contents of the anterior compartment of the knee-the fat pad (FP) and infrapatellar plica (IPP). The objective of this presentation is to describe an effective arthroscopic technique that treats AKP by addressing this common factor. The operation consists of release or resection of the IPP, or ligamentum mucosum, which tethers the FP. These highly innervated tissues act together as a hydraulic shock absorber, filling the anterior compartment. They stretch and deform at the extremes of knee motion because of constraint centrally by the non-isometric IPP. These dynamic changes in shape are eliminated when the plica is released or resected. Pain perception is from perturbed nociceptive nerves: pain relief results from de-tensioning these contained nerves by untethering the fat pad. Ascribing pain causation is problematic because morphologic change, such as inflammation, fibrosis, or contracture of these structures, is only present in a minority of cases. Nonetheless, AKP is both physically linked to these central, pain-sensitive structures and relieved by this operation.

Figures

Fig 1
Fig 1
Non-isometric mechanical behavior of infrapatellar plica (IPP), viewed arthroscopically (screenshots from Video 1). Arthroscopic views of the notch from the anterolateral portal (A-E) and anteromedial portal (F) are shown in the right knee of a 16-year-old patient with anterior knee pain for 4 years, with an insidious onset, undergoing release and resection of the IPP at its femoral attachment (FA). (A) At 90° of knee flexion, the IPP, of normal structure, is vertical and taut. (B) At 45° of knee flexion, the IPP has 2 parallel elements, classified as “split.” The leading edge is a curved arc suggesting laxity. (C) At 20° of knee flexion, the leading edge straightens as the IPP approaches the apex of the notch; the FA has rotated posteriorly out of view. (D) At 10° to 15° of knee flexion, the IPP contacts and indents the articular surface at the notch; with further extension, the apex of the notch acts as a pulley. From kinematic videos, the IPP increasingly elongates with further extension. The apposing aspect of the IPP and the central body attachment of the fat pad (FP) are compressed against the articular surface of the trochlea, potentially inducing surface reactive change. (E) At 0° of knee flexion and in full extension, the IPP and the central body attachment of the FP to the IPP have rotated out of view and the FP apposes the trochlear articular surface. (F) At 90° of knee flexion, with viewing from the anteromedial portal, the IPP has been released. The FP seen on the left, floating free of the central attachment, is no longer tethered by the IPP at the FA. (CB, central body attachment of FP to IPP; FA, femoral attachment; Infl, small zone of possible inflammation.)
Fig 2
Fig 2
Photograph of the left knee of a supine patient who has undergone release and resection of the infrapatellar plica for anterior knee pain. Usually the anterolateral portal (ALP) and anteromedial portals (AMP) are used. Local anesthetic (2 mL of 0.25% bupivacaine) is instilled at the site immediately before placement of any portal. The ALP is established first as the primary working portal. With the knee flexed 90°, the incision is made at the level of the lower pole of the patella, just lateral to the palpable lateral border of the patellar tendon. The AMP is selected using percutaneous placement of an 18-gauge spinal needle, entering just superior to the medial meniscus, avoiding the medial border of the fat pad. The superolateral portal (SLP) is established for viewing from above, as well as for more direct instrument access. It was not used in this case, but its location is marked, 2.5 to 3 cm above the superior pole of the patella, in line with its lateral border.
Fig 3
Fig 3
Kinematics of contents of anterior compartment, including infrapatellar plica (IPP)–fat pad (FP) complex (screenshots from Video 1). Radiographic contrast was placed into the IPP and FP of a volunteer undergoing arthroscopy for anterior knee pain. All views show the right knee from the anterolateral portal. (A) With approximately 90° of knee flexion, the IPP femoral attachment (FA) has a broad base and then narrows and disappears; it is hidden in the overlap of contrast of the FP. The overall shape of the FP is tubular. (B) With approximately 45° of knee flexion, the IPP is lax as shown by the ripple in its mid portion; more of the IPP is visible. The FP (tubular, broader, appearing foreshortened) seems to “point” to the IPP; there is a gap between the lower pole of the patella and the FP. (C) With approximately 5° of knee flexion, the IPP is linear and stretched. The former broad base is less visible; the contrast has been squeezed out of the FA. The FP is now irregular in shape, with a small gap between the FP and patella. (D) Maximal extension, with maximal effort at quadriceps contraction (quadriceps-set maneuver), combined with the relative changes in the position between the femur and tibia with the screw-home mechanism, produces remarkable stretch of the IPP and further distortion of the FP. The central body–transition zone IPP-FP (CB) is now seen, previously hidden by overlap of the medial and lateral extensions of the FP. In summary, best observed in B, the FP is pulled away from the lower pole of the patella by the IPP tether. Highly innervated, the IPP-FP complex stretches and distorts as the knee moves, tethered centrally by the IPP.
Fig 4
Fig 4
Kinematics of infrapatellar plica (IPP)–fat pad (FP) complex after release of IPP (screenshots from Video 1). All views show the right knee from the anterolateral portal. The central body and IPP are not seen; the FP has been untethered. (A) With approximately 90° of knee flexion, the FP appears quadrilateral, occupying the notch with medial and lateral extensions apposing the condyles. (B) With approximately 45° of knee flexion, the FP sits matching the curve of the articular surface of the femoral notch, with the superior border immediately adjacent to the patella above, not distracted by the central tether of the IPP. This image should be compared with Figure 3B. (C) With 5° of flexion, there is a similar appearance to B with the FP matching the curves of the distal femur. (D) At maximal extension, the FP again matches the contour of the trochlea and condyles, with the superior border apposing the patella tightly. In summary, the compliant, semiliquid, now-untethered FP simply fills the available space of the anterior compartment, with far less distortion. The mechanical perturbations of the enthesis organ have been abolished by untethering the FP.
Fig 5
Fig 5
In step 2, we view the apex of the notch, classify the infrapatellar plica (IPP), and note abnormalities. A right knee is shown, viewed through the anterolateral portal, in a 32-year-old male patient with anterior knee pain for 6 years after a direct blow to the front of the knee (second case presented in Video 1). (A) In this gestalt view in mid flexion, an attempt is made to visualize the complete functional unit of the IPP. There is a shallow groove at the apex of the notch and below, the IPP appears “separate”. (B) However, adjusting flexion to 30° gives the complete picture. The IPP is a wall of connective tissue, a vertical septum, in which there is an opening, or fenestra. Above, the femoral attachment of the IPP (FA)—an enthesis—is seen as a curved arc of connective tissue attached to bone. Below and behind is the now relaxed border of the fenestra, connective tissue that merges with the anterior border of the ACL. Adjacent to the IPP is a separate connective tissue band, an anomaly. The IPP is thus a fenestrated vertical septum. (C) At about 60° the more dense superior border of the IPP is apparent, with diaphenous loose connective tissue at the margin of the window. (D) Close-up of the fenestra, with the ACL behind, the rest of the IPP above, and fat pad to the right. (ACL, anterior cruciate ligament.)
Fig 6
Fig 6
In step 3, we evaluate the mechanical behavior of the infrapatellar plica (IPP). A fenestrated vertical septum is present. A right knee is shown, viewed through the anterolateral portal. (A) With the knee at approximately 90°, the IPP is vertical and the fenestra is distorted. (B) With the knee at approximately 45°, the leading edge of the IPP is straight; attachment to the underlying anterior cruciate ligament attenuates the mechanical behavior—laxity is not seen. (C) With the knee at approximately 15°, the IPP, under increasing stretch (non-isometric plus screw-home mechanism), contacts the articular border at the apex of the notch, which becomes a pulley. Tension in the fat pad is arising as well from the proximal pull of the extensor apparatus; tension in the IPP is arising from non-isometricity plus posterior rotation of the femoral attachment. (D) With the knee at approximately 15°, the IPP is no longer seen; the fat pad is apposed to the trochlear articular surface. Under increasing tension as shown in Figure 9, the complex tissues arrayed in the IPP, including dense and loose connective tissue, elastin, and fat, merge with direct dense connective tissue bundles connecting to the lower pole of the patella (Fig 10B). This connective tissue network mechanically acts as an increasingly loaded cable, a construct of remarkable strength, a principle used in architecture, wherein a floppy cable, of the right material, when anchored from a firm base becomes increasingly more rigid, capable of supporting bridges, roofs, and walls.
Fig 7
Fig 7
In step 4, untethering of the fat pad is performed by release and resection of the infrapatellar plica (IPP) and restoration of the anatomy of the anterior compartment. (A) A carefully positioned punch is used to release the femoral attachment (FA). (B) The punch is used to detach the connective tissue elements adherent to the underlying anterior cruciate ligament (ACL). (C) The shaver is introduced, with the hood reversed, to tease the connective tissue from the ACL, working from proximal to distal. (D) The surgeon should stop at the intermeniscal ligament. The abnormality—an anomalous fibrous band—is removed using the shaver. This completes the untethering of the fat pad. Other abnormalities are not often present, but if encountered, the combination of a punch, soft-tissue shaver, burr, and radiofrequency device is used to remove and restore the anatomy.
Fig 8
Fig 8
The surgeon should review and verify that the fat pad has been untethered and the borders of the anterior compartment restored. A right knee is shown, viewed through the anterolateral portal. (A) In the gestalt view, the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are intact; the anomalous band and the infrapatellar plica (IPP) have been removed. The fat pad, now with no connection to the distal femur or structures in the notch and displaced by fluid pressure, is out of view. (B) Close-up of the apex of the notch. Again, the fat pad is displaced. The cartilage and underlying bone at the apex of the notch have regressively remodeled, creating a groove in response to the mechanical forces applied by the leading edge of the IPP, with contact beginning in terminal extension (15°-20°). Remaining strands of connective tissue mark the location of the femoral attachment (FA)—the entheseal attachment of the IPP (histology in Fig 11 C and D). (C) In the gestalt view with the anterior compartment below, the trochlear articular surface shows linear erosion along its length, the fat pad is again untethered and displaced, and the patella above shows grade 2 chondral surface changes.
Fig 9
Fig 9
Artist's sketches showing the biomechanical effect of the infrapatellar plica (IPP)–fat pad (FP) complex. (A) The combined effect of the screw-home mechanism (in which the femoral attachment moves relatively posteriorly and the tibia moves anteriorly and rotates externally, approaching full extension), and isometricity, induce increasing stretch of the IPP and distortion of the FP, perturbing the contained neural network. As the knee approaches terminal extension, tension increases within the IPP-FP complex, as they are, as a functional unit, compressed increasingly against the trochlear articular surface. Shear and compression forces on the articular cartilage may lead to morphologic change, in the form of regressive remodeling (linear grooves are often seen in the trochlea attesting to this) (Fig 8C). Frank osteoarthritic change clearly could follow if the forces are excessive or of long duration. The extensor apparatus is linked to the IPP-FP complex through the dense connective tissue connections of the synovial layer, in essence a force-transmitting and attenuating array (Fig 11B). The kinematic studies show even more stretch and deformity with the quadriceps-set maneuver. (B) At 45°, the studies show that the separate plica is lax (Fig 3B) such that the IPP can be easily stretched with a probe and has minimal tension within it.
Fig 10
Fig 10
Anatomic relations of the anterior compartment (AC), a space whose borders change with motion. (A) A midline sagittal section shows the contents of the AC (bordered in red). Structured to be semiliquid and deformable, the fat pad (FP) fills the space, tethered to the femur at the femoral attachment (FA) by the infrapatellar plica (IPP), a ligament, which is present in most patients (86% depending on population). The composite structure—FP, IPP, and contained deep infrapatellar bursa—is defined as a functional unit, the IPP-FP complex, an enthesis organ, whose task is filling space and attenuating force. The key biomechanical concept is that the FA (an enthesis) is located inferior to the instant center of rotation of the knee (ICR), making the IPP-FP complex non-isometric. With motion, contained within the borders of the AC, the IPP-FP complex, tethered at the FA, rotates around it and not the ICR (Fig 3, Video 1), leading to stretch and deformity. (B) Sagittal section of an anatomic specimen showing how the AC (bordered by the dashed line) is filled by the IPP-FP complex. The probe is in the infrapatellar bursa. (C) Magnetic resonance imaging correlate, with the knee in full extension. The IPP-FP complex fills the AC and is in tight apposition to the articular surface of the trochlea. (CB, central body [which links FA and FP].)
Fig 11
Fig 11
Gross anatomic relationships of fat pad (FP) and infrapatellar plica (IPP) in 2 specimens (A, B) and histology of enthesis (EN) (C, D). The extensor apparatus has been released from the femur and flipped 180°, with the femur above and the patella below. (A) The FP elements are the conical central body (CB), the FP transition zone to the IPP; the extensions, comprising the medial extension (ME) and lateral extension (LE) of the FP, which appose the condyles; and the alar folds (AFs), which are connective tissue links to the synovial layer elements above. The IPP elements include the femoral attachment (FA), an EN, the transition zone attachment to bone, ligamentous central zone (CZ), and CB transition zone to the FP. (B) Specimen from a cachectic donor, showing little remaining fat, thus allowing the underlying dense connective tissue (DCT) array to be seen. The DCT from all quadrants coalesces at the CB to merge with the DCT of the CZ; inferiorly, the inferior bands (IB) attach to the tibia, intermeniscal ligament, and anterior horns of the menisci; superiorly, on either side, the AFs merge with the parapatellar folds, and centrally, thick superior bands (SB) attach to the lower pole of the patella (mechanical role in full extension). The DCT network transmits and attenuates force; the compressible fat, in septate lobules, attenuates force. (C, D) Photomicrographs of FA, both stained with Hematoxylin and Eosin, confirming that it is an EN, a transition zone attenuating force. (C) 20 × original magnification. Overview of the FA at the apex of the notch. Articular cartilage (AC) merges with bone at the lower left with the loose connective tissue (LCT) above, bordering the DCT, at upper right. Force attenuation (function) correlates with tissue structure at the EN, in which each layer is less rigid, with bone as the anchor. (D) 100 x original magnification of EN. The zones are (1) cortical bone below; (2) The tidemark (TM), a densely calcified, irregular anchoring structure; (3) a second irregular calcified line beginning at MF, which marks the adjacent region of mineralized and unmineralized fibrocartilage, containing chondrocytes (CC) in lacunae; and (4) the DCT of the ligamentous IPP above.

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