CHAPTER SYNOPSIS
In this chapter, we discuss the current status of patellofemoral (PF) arthroplasty. The background, including indications, and appropriate workup are described, as well as design issues and their impact on clinical outcomes. Technical points on implantation of PF devices are outlined.
IMPORTANT POINTS
The reader’s attention is brought to understanding the role of correct patient selection in the success of PF arthroplasty. Good preoperative and postoperative investigation and care are discussed. Correct surgical technique is described, and historical improvement with implant design is discussed. An outline of alternative techniques as well as future developments is also offered.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
Patellofemoral (PF) arthritis is a relatively common condition. It has been shown to affect up to 11% of men and 24% of women over the age of 55 years. Isolated PF arthritis has been demonstrated in 9.2% of symptomatic knees over the age of 40. The disease appears to affect women predominantly. The typically larger Q angle found in females is thought to increase the propensity to knee and PF disorders due to its effect on both PF and tibiofemoral kinematics. Despite some studies demonstrating no gender-related difference in kinematics, women dominate the study cohorts of most published data on PF arthroplasty.
PF arthritis may be defined as loss of articular cartilage on one or both surfaces of the patella and/or trochlear groove. Predisposing factors include previous trauma, inflammatory arthritides, and abnormal biomechanics around the knee.
The PF joint offers challenges because of the unique arrangement of static (ligaments and bony elements) and dynamic (neuromuscular) factors that contribute to its operation. These include the medial and lateral PF and patellotibial ligament complexes, quadriceps function, and limb alignment. The latter involves coronal, axial, and sagittal tibiofemoral alignment, femoral version, and pelvic position. This complex orchestration of forces must be correctly interpreted and understood to avoid inappropriate surgical intervention.
Nonarthroplasty surgical management of PF arthritis includes realignment procedures, cartilage replacement (autologous chondrocyte transplantation and mosaicplasty), and arthroscopic releases and debridement, as dealt with elsewhere in this volume. Total knee arthroplasty (TKA) is a proved method for the treatment of PF arthrosis in the elderly patient. For the purposes of this chapter, however, we concern ourselves primarily with unicompartmental PF arthroplasty.
Understanding the forces around the PF joint is important for correct implantation technique. PF arthroplasty has the potential advantage of retaining the menisci, anterior, and posterior cruciate ligaments and thereby the natural kinematics of the knee joint. The shortcoming of this procedure to date has been the durability of these arthroplasties. This is discussed later.
The patella has two main functions: to improve the mechanical advantage of knee extension by increasing the effective lever arm of the quadriceps muscle group and to reduce the contact forces in the PF joint and tibiofemoral joints. Alignment of the soft tissues is critical to stability of the PF joint. Tension of the quadriceps exerts a force tending to displace the patella laterally. This is restricted anatomically by the prominent lateral femoral trochlear facet and an opposite force exerted by the vastus medialis obliquus, which inserts almost horizontally into the patella. Additional stability is provided by the medial retinacular structures, primarily the medial PF ligament.
The Q angle is defined as the angle between a line joining the superior anterior iliac spine and the center of the patella and the projection of a line from the tibial tuberosity through the center of the patella. Any condition that increases this angle results in an increased lateral force that may cause subluxation or dislocation. Abnormal angles are considered greater than 15 degrees in males and 20 degrees in females, and this angle is physiologically increased at terminal extension as the tibia externally rotates to lock into place.
Patellofemoral joint reaction (PFJR) force is dependent on the tensions created by the extensor mechanism that loads the patella against the femur. This force increases as the knee flexes, as does the PF contact area, at least until 60 degrees of flexion. The joint reaction force passes through different areas of the patella as the knee bends. With the knee slightly flexed, it passes through the lower portion and migrates proximally with further flexion as the patella translates distally. Experimental models have helped determine that the PFJR force is up to 3.3 times body weight at 60 degrees flexion (climbing stairs) and 7.8 times body weight at 130 degrees (deep knee bends).
Perhaps due to its complex origin, PF arthrosis has presented a therapeutic dilemma across the spectrum of disease from abnormal loading to advanced wear. Nonarthroplasty options for PF arthrosis have met with variable results. TKA has good results, but 7% to 19% of patients experience residual anterior knee pain when TKA is performed for PF arthritis in isolation.
While PF arthroplasty is not a new concept, its appropriate indication has yet to be established and is currently a matter of considerable debate. It has generated much interest as a bone-sparing surgical option in for advanced PF arthritis. One of the major factors leading to caution surrounding the application of PF arthroplasty is the relatively poor performance of early designs. These will be discussed later.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
Patellofemoral (PF) arthritis is a relatively common condition. It has been shown to affect up to 11% of men and 24% of women over the age of 55 years. Isolated PF arthritis has been demonstrated in 9.2% of symptomatic knees over the age of 40. The disease appears to affect women predominantly. The typically larger Q angle found in females is thought to increase the propensity to knee and PF disorders due to its effect on both PF and tibiofemoral kinematics. Despite some studies demonstrating no gender-related difference in kinematics, women dominate the study cohorts of most published data on PF arthroplasty.
PF arthritis may be defined as loss of articular cartilage on one or both surfaces of the patella and/or trochlear groove. Predisposing factors include previous trauma, inflammatory arthritides, and abnormal biomechanics around the knee.
The PF joint offers challenges because of the unique arrangement of static (ligaments and bony elements) and dynamic (neuromuscular) factors that contribute to its operation. These include the medial and lateral PF and patellotibial ligament complexes, quadriceps function, and limb alignment. The latter involves coronal, axial, and sagittal tibiofemoral alignment, femoral version, and pelvic position. This complex orchestration of forces must be correctly interpreted and understood to avoid inappropriate surgical intervention.
Nonarthroplasty surgical management of PF arthritis includes realignment procedures, cartilage replacement (autologous chondrocyte transplantation and mosaicplasty), and arthroscopic releases and debridement, as dealt with elsewhere in this volume. Total knee arthroplasty (TKA) is a proved method for the treatment of PF arthrosis in the elderly patient. For the purposes of this chapter, however, we concern ourselves primarily with unicompartmental PF arthroplasty.
Understanding the forces around the PF joint is important for correct implantation technique. PF arthroplasty has the potential advantage of retaining the menisci, anterior, and posterior cruciate ligaments and thereby the natural kinematics of the knee joint. The shortcoming of this procedure to date has been the durability of these arthroplasties. This is discussed later.
The patella has two main functions: to improve the mechanical advantage of knee extension by increasing the effective lever arm of the quadriceps muscle group and to reduce the contact forces in the PF joint and tibiofemoral joints. Alignment of the soft tissues is critical to stability of the PF joint. Tension of the quadriceps exerts a force tending to displace the patella laterally. This is restricted anatomically by the prominent lateral femoral trochlear facet and an opposite force exerted by the vastus medialis obliquus, which inserts almost horizontally into the patella. Additional stability is provided by the medial retinacular structures, primarily the medial PF ligament.
The Q angle is defined as the angle between a line joining the superior anterior iliac spine and the center of the patella and the projection of a line from the tibial tuberosity through the center of the patella. Any condition that increases this angle results in an increased lateral force that may cause subluxation or dislocation. Abnormal angles are considered greater than 15 degrees in males and 20 degrees in females, and this angle is physiologically increased at terminal extension as the tibia externally rotates to lock into place.
Patellofemoral joint reaction (PFJR) force is dependent on the tensions created by the extensor mechanism that loads the patella against the femur. This force increases as the knee flexes, as does the PF contact area, at least until 60 degrees of flexion. The joint reaction force passes through different areas of the patella as the knee bends. With the knee slightly flexed, it passes through the lower portion and migrates proximally with further flexion as the patella translates distally. Experimental models have helped determine that the PFJR force is up to 3.3 times body weight at 60 degrees flexion (climbing stairs) and 7.8 times body weight at 130 degrees (deep knee bends).
Perhaps due to its complex origin, PF arthrosis has presented a therapeutic dilemma across the spectrum of disease from abnormal loading to advanced wear. Nonarthroplasty options for PF arthrosis have met with variable results. TKA has good results, but 7% to 19% of patients experience residual anterior knee pain when TKA is performed for PF arthritis in isolation.
While PF arthroplasty is not a new concept, its appropriate indication has yet to be established and is currently a matter of considerable debate. It has generated much interest as a bone-sparing surgical option in for advanced PF arthritis. One of the major factors leading to caution surrounding the application of PF arthroplasty is the relatively poor performance of early designs. These will be discussed later.
INDICATIONS/CONTRAINDICATIONS
As in all aspects of surgical practice, correct patient selection is essential to good outcome. Following failed conservative management, PF arthroplasty is indicated in PF arthrosis secondary to trauma and in primary PF arthritis where tibiofemoral arthrosis has been ruled out. Revision to TKA because of tibiofemoral disease progression represents a significant proportion of revised PF arthroplasties. Although established arthrosis of the tibiofemoral joint is a contraindication for PF arthroplasty, there have been recent advocates for combining grafting of isolated full-thickness tibiofemoral chondral lesions with PF arthroplasty. However, there are no published data on this approach, and it should be approached with caution pending further evaluation of the technique.
PF arthroplasty has been described as an effective treatment for PF dysplasia. Its use in cases of significant malalignment including large Q angle (see later) is less promising and should be avoided unless the malalignment is also corrected. Mild patellar subluxation or tilt can be managed at the time of PF arthroplasty with a lateral retinacular release and careful implant positioning.
Given higher rates of tricompartmental arthrosis and the predictable results of TKA in older age groups, PF arthroplasty tends to be considered as a relatively bone-sparing procedure for patients younger than 55 years with isolated PF compartment arthrosis. Evaluation of more recent implants has included older age groups.
Patient Assessment
History
Patterns of pain in PF arthritis are typically anterior or retropatellar knee pain with increasing discomfort on descending or, particularly, climbing stairs. Mobilization is more difficult on uneven ground. The patients prefer to sit with their knees extended rather than flexed. There may be difficulty squatting or kneeling. The patient may describe crepitus and swelling. Particular attention should be paid to a history of patellar subluxation or dislocation as this could indicate the need for a realignment procedure in advance of PF arthroplasty. A history of previous trauma should also be sought.
Physical Examination
The patient is exposed adequately to reveal both lower limbs including access to the anterior superior iliac spine. Any obvious deformity in coronal and sagittal alignment is noted, with particular attention paid to determining the Q angle ( Fig. 6-1 ). A value of greater than 15 degrees in males and 20 degrees in females indicates the need for corrective surgery before undertaking PF arthroplasty. Any previous scars should be noted as they may compromise access and the neurovascular and dermatological condition of the limb should be noted. Lateral subluxation of the patella in the terminal 20 degrees of extension is known as the J sign and is an indicator of PF instability, malalignment, and muscle imbalance, which should be addressed before undertaking PF arthroplasty.
An effusion should be identified if present, and pain elicited on patellar tap is a good indicator of PF pathology. The patella should be examined for stability, and a positive apprehension test may indicate the requirement for aggressive physiotherapy or realignment procedures. Tenderness along the lateral and medial joint lines may indicate arthrosis affecting the tibiofemoral compartments. Any tenderness in the quadriceps or patellar tendons and hamstring or iliotibial band insertions should be ruled out. Anterior or posterior cruciate ligament insufficiency is not an absolute contraindication for this procedure, but ligament reconstruction should be actively considered to reduce instability and possibly help preserve tibiofemoral cartilage.
Examination of the spine, hips, and feet is important. The “miserable malalignment syndrome” consists of coexisting internal torsion of the femur, external torsion of the tibia, and planovalgus foot. It is strongly associated with PF pain and may be amenable to improvement with orthotics.
Radiology
Standard anteroposterior and lateral radiographs should be taken while weight bearing to avoid underestimation of tibiofemoral disease. Rosenberg views are weight-bearing flexed views in the plane of the tibial plateau that identifies tibiofemoral contact in flexion ( Fig. 6-2 ). Merchant views (skyline views at 30 degrees, Fig. 6-3 ) are also helpful. Lateral radiographs can demonstrate patella baja, which should be corrected before PF arthroplasty, but they are not reliable in the determination of site and degree of disease and should always be correlated with the other more specific views. Full leg-length standing radiographs can aid assessment of mechanical axial alignment. It may be necessary to consider computed tomography scanning or magnetic resonance imaging to fully assess PF tracking and quality of articular cartilage respectively.
Related posts:
Sizing and Balancing: Gap Technique versus Measured Resection
Addressing Ligament Deficiency in Revision Total Knee Arthroplasty
Removal of Well-Fixed Components during Revision Total Knee Arthroplasty
Extensor Mechanism Rupture
Adjunctive Fixation in Total Knee Arthroplasty Revision: Stems and Sleeves
Surgical Exposure in Total Knee Arthroplasty
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