Patellofemoral Degeneration and Patellar Instability
Jack Farr
Vishal S. Desai
Diane L. Dahm
INTRODUCTION
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.
I. BIPOLAR PATELLOFEMORAL CARTILAGE RESTORATION
Pathogenesis
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.
Classification
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.
SURGICAL ANATOMY
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.
Evaluation
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
Imaging
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. |
Differential Diagnosis
Advanced chondrosis versus arthrosis is in the eyes of the beholder as ICRS grade 3C chondrosis is exposed bone.
Is this unicompartmental disease or just the first site of symptomatology in a knee that actually has bicompartmental or tricompartmental disease?
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.
Positioning
Supine position with limb free to assess intraoperative full range of motion.
Padding under ipsilateral hip to aid in neutral limb rotation.
Approach/Exposures
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 | ||||
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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).
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.
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).
Closure/Dressing
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 26.11 Osteochondral allograft patellar shell showing standard compressive fixation with bioabsorbable screws. |
PEARLS AND PITFALLS OF BIPOLAR PATELLOFEMORAL CARTILAGE RESTORATION | ||||
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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