Patellofemoral Degeneration and Patellar Instability

Patellofemoral Degeneration and Patellar Instability

Jack Farr

Vishal S. Desai

Diane L. Dahm


  • For the purposes of this chapter, patellofemoral (PF) degeneration is used to denote chondrosis that is more extensive than focal monopolar chondral lesions, as discussed in Chapter 25.

  • There are 3 techniques described in this chapter:

    • Bipolar Patellofemoral Cartilage Restoration

    • Lateral Patellofemoral Facetectomy

    • Patellofemoral Arthroplasty.



  • Earlier chapters discuss the multifactorial nature of recurrent patellar instability (RPI).

  • PF degeneration is the end result of a combination of these factors over time.

  • Radiographic evidence of PF degeneration (with or without RPI) can be observed in up to 17.1% to 34% of female patients and 19% of male patients by the end of middle age.1

  • Recurrent patellar dislocations typically occur in the young and are often associated with trochlear dysplasia, which even when not associated with patellar instability, carries an increased risk of PF cartilage deterioration. In one series, 78% of knees with PF degeneration were dysplastic.2

  • In the nondysplastic patients, the cascade of chondrosis progression is related to both the trauma of dislocations (commonly involving the distal medial patellar facet with a subset of patients having involvement lateral to the median ridge) and the chronic lateral position of the patella in the trochlear groove, which is often the result of chronic patholaxity of the medial PF ligament complex (the wear pattern of chronic patellar subluxation involves the lateral facet of the trochlea and the articulating surface of the patella to the degree of subluxation; may range from medial ridge to pan-patellar).

  • As chondral damage progresses, the increased coefficient of friction will increase the resistance to dislocation forces and eventually may even lead to resolution of the RPI.

  • There is a small subset of patients with a genetic predisposition to osteoarthritis. Although the treatment is the same, identifying them preoperatively with family history and examination of the tibiofemoral (TF) compartments and limb alignment will aid in discussing their prognosis.


  • Merchant classified PF degeneration into four stages based on low flexion angle axial radiographs.3

    • Stage 1: Mild with more than 3 mm of joint space.

    • Stage 2: Moderate with less than 3 mm of joint space, but no bony contact.

    • Stage 3: Severe with bony surfaces in contact over less than one-quarter of the joint surface.

    • Stage 4: Very severe with bony contact throughout the entire joint surface.

  • Even low flexion angle radiographs typically underestimate the degree of chondral damage as well as the degree of trochlear dysplasia. That is, even stage 1 PF compartments may have extensive bipolar chondrosis.

  • Currently, magnetic resonance imaging (MRI) or computed tomography (CT) arthrogram is the standard of care to evaluate the specific depth and area of articular cartilage damage.

  • For both CT and MRI, the standard images are orthogonal to the femoral shaft and tibia, but not to the trochlea. This results in trochlear images that are oblique cuts that may be difficult to interpret accurately. Therefore, when evaluating these patients, specific cuts that are orthogonal to the trochlear morphology are useful.4

  • To be placed in the PF degenerative category, the lesions are:

    • Bipolar

    • Involve greater than 50% of the cartilage thickness (International Cartilage Repair Society [ICRS] equivalent grades 3 and 4)5

    • Surface areas of both are greater than 2 cm2.


  • Extensive but contained bipolar PF chondrosis (Figure 26.1).

  • Extensive uncontained bipolar PF chondrosis (Figure 26.2).

  • Table 26.1 lists indications and contraindications for bipolar patellofemoral cartilage restoration.


Patient History

  • Patients relate a history of RPI in their childhood to early 20s with minimal pain between episodes.

  • In their 20s, the RPI decreases as they learn to cope/avoid patellar instability and they note gradual increases in pain with PF loading activities.

  • In their late 20s to 30s, they gradually have increased pain with PF loading to the point that it interferes with their quality of life. At this point, they may or may not have RPI.

Physical Findings

  • Standard core-to-floor evaluation of strength, balance, and alignment including coronal and axial alignment

  • Standard knee examination

  • Standard PF examination for areas of tenderness, patellar height, mobility, and apprehension


  • Full radiographic series to include standing anteroposterior (AP), flexed posteroanterior (PA), true lateral, low flexion angle axial view (Figure 26.3), and standing long leg hip-to-ankle alignment view.

  • MRI with optional trochlear oblique sections and to include T2 fat-suppressed images to better evaluate subchondral bone stress reactions and cystic changes; MRI also thoroughly assesses the TF compartment for pathology.

  • Thin section CT arthrogram when MRI images are insufficient for surgical decision.

Other Studies

  • Staging arthroscopy to precisely grade and map the lesions and thoroughly assess patellar tracking as well as TF compartment status.

  • While in operating room, assess patellar mobility under sedation to grade displacement, subluxation, or frank dislocation.

Figure 26.1 A, Bipolar contained lesions of patella and trochlea, predebridement. B, Lesion after defect preparation to vertical walls and clear base.

Figure 26.2 Bipolar uncontained lesions of patella and trochlea.

Nonoperative Management

  • Standard core-to-floor PF rehabilitation program.

  • Trial of PF bracing.

  • Trial of non-narcotic oral analgesics and nutritional supplements as indicated by current literature.

  • Trial of corticosteroid, viscosupplement injections. Possible biologic injections as per interpretation of the literature (platelet-rich plasma, bone marrow aspirate/concentrate, etc.).

  • Activity modification.

  • Environmental modifications, for example, home with no stairs, avoid inclines, and steps outdoors.

  • Weight loss for those with excess body mass index (BMI).

Surgical Management

Preoperative Planning

  • Use the information gathered at staging arthroscopy to decide on cell therapy (autologous cultured chondrocyte implantation [ACI], or with matrix autologous chondrocyte implantation [MACI]) versus osteochondral allograft (OCA).

  • Because private and governmental insurance may have specific policies regarding these procedures, it is wise to request formal preauthorization permission for the proposed surgery.

  • From MRI and/or CT arthrogram, decide on the need for tuberosity osteotomy: straight anteriorization or anteromedialization with or without a component of distalization.

  • Imaging and examination to decide on the probable need for lateral lengthening or lateral release.

  • Use the examination under anesthesia findings to plan for medial patellofemoral ligament (MPFL) shortening versus formal MPFL reconstruction.


  • Supine position with limb free to assess intraoperative full range of motion.

  • Padding under ipsilateral hip to aid in neutral limb rotation.


  • Midline skin incision to allow for lateral lengthening and MPFL surgery.

  • To aid in the MPFL surgery portion, dissect between layers 2 and 3 and establish the tunnel as needed to the femoral attachment region. Tag layers 1 and 2, then enter layer 3 and synovium.

TABLE 26.1 Indications and Contraindications for Bipolar Patellofemoral Cartilage Restoration



  • Failed nonoperative management

  • Bipolar patellofemoral chondrosis of >2 cm2 in area and >50% chondral loss

  • May currently have recurrent patellar instability or maybe only historical

  • Acceptable comfort and function

  • Monopolar lesions that may be amenable to standard cartilage restoration options (see Chapter 25)

  • Similar bipolar cartilage pathology, but in a patient who is age appropriate for patellofemoral arthroplasty

Figure 26.3 Low flexion angle axial view. This view provides assessment of patellofemoral morphology and alignment, but often underestimates trochlear dysplasia and subluxation. This patient eventually had bipolar osteochondral allograft plugs with concomitant anteromedialization and lateral lengthening.

Procedural Steps

Cell Therapy With Matrix

  • If concomitant tuberosity surgery is planned, perform osteotomy first to improve ease of visualization.

  • Medial or lateral arthrotomy to allow optimal lesion exposure.

  • Use a #15 scalpel to create vertical walls of normal/near normal cartilage and clear the base to, but not into, the subchondral bone plate. When necessary, prepare to use suture-through-bone or mini suture anchors for regions of uncontainment (Figure 26.4).

  • Release the tourniquet if it had been inflated. Achieve hemostasis.

  • Use standard technique to apply MACI into the prepared lesion as per manufacturer (Vericel, Cambridge, Massachusetts), noting as MACI is new in the United States, the examples provided here are ACI. The only difference is the cells are grown on the collagen patch for MACI and often allow fibrin glue fixation alone without the need for sutures used for ACI.

    • Because this is a bipolar application, the patches of MACI and/or ACI will articulate at some point during range of motion, and therefore, rather than rely on fibrin glue to maintain implant position, augment the glue fixation with sutures similar to previous ACI techniques (Figure 26.5).

    Figure 26.4 A, Defect preparation with vertical walls and clear base. B, Uncontained bipolar lesions treated with autologous chondrocyte implantation; note the four white suture anchors placed at uncontained margins of the patellar lesion and one at the trochlear lesion.

    Figure 26.5 For bipolar applications, use sutures and/or suture anchors similar to previous autologous chondrocyte implantation technique. Owing to the articulating nature of bipolar patches, additional sutures will help maintain fixation through range of motion.

  • Perform reduction and fixation of tuberosity if osteotomy has been performed. Repair the lateral retinaculum at the lengthened position.

  • Complete the MPFL shortening or formal reconstruction using soft-tissue fixation at the patella (suture anchors or soft-tissue envelope) (Figure 26.6).

Osteochondral Allograft

  • If concomitant tuberosity surgery is planned, do not perform osteotomy at this time because a secure patellar tendon will improve patellar stability for bone preparation.

  • Medial or lateral arthrotomy to allow optimal lesion exposure.

  • Size the lesions to assess whether they can be treated with plug-in-socket technique or whether a shell technique will be necessary.

    Figure 26.6 Medial patellofemoral ligament fixation at soft tissue of the patella.

    • For plug technique, create shallow sockets perpendicular to the local topology (±6 mm deep; that is, deep enough for bone fixation, yet shallow as allowed to minimize the allograft bone “immunologic load” on the knee) (Figure 26.7).

    • For shell technique, cut the patella to achieve a uniform thickness of more than 12 mm and a trochlear cut that minimizes bone removal (Figure 26.8).

  • Remove marrow elements with pulsatile lavage. Dry the cancellous bone and augment with bone marrow aspirate/concentrate (Figure 26.9).

  • Insert the plugs without impaction. If they are secure, no fixation is necessary; if the plugs are not fully secure, augment with bioabsorbable fixation (Figure 26.10).

  • For OCA shells, secure with standard compressive fixation (Figure 26.11).

  • Perform tuberosity osteotomy if planned. Repair the lateral retinaculum at the lengthened position.

  • Complete the MPFL shortening or formal reconstruction using soft-tissue fixation at the patella (suture anchors or soft-tissue envelope) (Figure 26.6).


  • Close in layers

  • Compressive dressing

  • Protective brace in full extension

Alternate Techniques

  • An alternative to shells technique, macro-plug is an option. Typical plug sizes range in diameter from 14 to 22 mm; however, Dr Thomas Deberardino, Dr Raffy Mirzayan, and others have had success using this technique with significantly larger plugs (25-30 mm) that replace nearly the entire articulating surface of the patella and/or the trochlea (Figure 26.12).

Figure 26.7 A, Preparation of osteochondral allograft patellar socket. B, Comparing median ridge of donor to patient for orientation of plug harvest site. C, Osteochondral allograft plug. D, Osteochondral allograft plug implanted.

Figure 26.8 A, For patellar shell technique, an oscillating saw is used to cut the patella in a manner similar to patellofemoral arthroplasty. The remaining bony portion has a thickness of approximately 11 to 14 mm. B, The oscillating saw is used to confirm that the patellar cut is a truly level plane.

Figure 26.9 A, Pulsatile lavage is used to remove marrow elements from the donor graft. B, Patella is cut in a similar manner with medial and lateral composite thickness as thin as possible (5-6 mm) to not overstuff the compartment because the median ridge composite thickness is approximately 10 mm for a total thickness of 22 to 24 mm, which is slightly greater than a native patella. Bone marrow aspirate harvested from the iliac crest is applied to the entire graft interface of both donor and patient.

Figure 26.10 Osteochondral allograft patellar plug shown with bioabsorbable screw fixation.

Figure 26.11 Osteochondral allograft patellar shell showing standard compressive fixation with bioabsorbable screws.




  • Temper patient expectations

  • Rapid progression of postoperative range of motion

  • Avoid PF loading during rehabilitation per type of implant

  • Bipolar PF OCA failure may be 40% or more at 5 y6,7

  • Early PF loading can lead to failure for both techniques

  • Postoperative course and patient satisfaction levels are lower than for patellofemoral arthroplasty (PFA)

Figure 26.12 A-D, Drs Thomas Deberardino and Raffy Mirzayan have also had success using a mega osteochondral plug approach to replace nearly all of the trochlea or patella. (Patella plug images kindly provided courtesy of Dr Thomas Deberardino. Trochlear plug images kindly provided courtesy of Dr Raffy Mirzayan.)

Postoperative Course

  • Protected TF weight bearing if tuberosity surgery

  • Weight bearing as tolerated in braced full extension if no tuberosity surgery

  • Aggressive increases in range of motion with continuous passive motion machine if available

  • Early thorough core-to-floor rehabilitation

Dec 1, 2019 | Posted by in ORTHOPEDIC | Comments Off on Patellofemoral Degeneration and Patellar Instability
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