What Predicts Functional Outcome After Treatment for Patellofemoral Pain?

Els Pattyn; Nele Mahieu; James Selfe; Peter Verdonk; Adelheid Steyaert; Erik Witvrouw

Disclosures

Med Sci Sports Exerc. 2012;44(10):1827-1833. 

In This Article

Abstract and Introduction

Abstract

Purpose: Although physical therapy is known to be effective in treating patellofemoral pain (PFP), there is considerable individual variation in the treatment response. It is unclear why some patients benefit from a specific treatment while others do not experience improvement. This study, using a prospective study design, aims to identify factors that could predict the short-term functional outcome and account for the variation frequently seen in the outcome after conservative treatment of PFP.
Methods: Thirty-six patients (20 female and 16 male with a mean age of 23.8 ± 6.7 yr) followed a physical therapy rehabilitation program of 7 wk. Before this treatment, all patients were evaluated on subjective symptoms (pain on visual analog scales in millimeters) and functional performance (step test expressed as highest level, single-legged hop test in centimeters, and triple-hop test in centimeters). The concentric and eccentric knee extensor strength at 60°·s−1 and 240°·s−1 (N·m) were measured as well as the quadriceps muscle size by calculating the cross-sectional area (cm2) with magnetic resonance imaging. The success of the treatment was evaluated by the functional Kujala anterior knee pain scale. A linear regression model was used to identify predisposing factors for the functional outcome.
Results: The total quadriceps cross-sectional area (P = 0.010), the eccentric average peak torque at 60°·s−1 (P = 0.015), and the frequency of pain at baseline (P = 0.012) have been indicated as predisposing variables in the short-term functional outcome after a physical therapy rehabilitation program for PFP (adjusted R2 = 0.46).
Conclusion: Patients with a greater quadriceps muscle size, lower eccentric knee strength, and less pain have a better short-term functional outcome after conservative treatment for PFP.

Introduction

Patellofemoral pain (PFP) is known as a common and often chronic musculoskeletal condition, affecting young and physically active adults.[12,25] PFP may account for almost 10% of all visits to outpatient sports clinics.[20] The exact etiology is still unknown but has been proposed to be multifactorial.[10,14] Because of the diverse origin of PFP, many rehabilitation programs with various emphases have been proposed to treat this disorder.

Several studies have demonstrated that physical therapy is effective in treating PFP.[8,9,28,38] According to the review of Crossley et al.,[9] there appears to be a consistent improvement in short-term pain and function as a result of physical therapy treatment. However, a third to a quarter of patients with PFP persist in having PFP. Nimon et al.[29] reported that one in four of their patients with anterior knee pain continued to have symptoms such as pain and disability at a mean follow-up of 16 yr after nonoperative treatment. In a prospective study of military recruits, 35% of the affected knees were still mildly, moderately, or severely painful at 6 yr of follow-up.[27]

It seems unclear why some patients benefit from a specific treatment although others do not experience improvement. Therefore, several attempts have been made to determine which factors may predict the outcome and account for the variation often seen in the conservative treatment of PFP.[7,21,28,33,39] These few prospective studies showed no consensus in identified prognostic factors. Collins et al.[7] and Witvrouw et al.[39] both indicated that patients with a long duration of knee pain had poorer prognosis after a treatment program. Natri et al.[28] demonstrated that the isometric extensor strength of the affected knee was a significant predictor of the functional outcome scores: the smaller the strength difference between the affected and the unaffected knee, the better the outcome. Other predictors of functional outcome were age and reflex response time of the vastus medialis obliquus (VMO) muscle.[21,39]

Studies identifying predictors of outcome mainly investigated pain, function, muscle strength and length, and biomechanical and clinical factors. To our knowledge, no attention has been paid to physiological characteristics such as quadriceps muscle size. Because atrophy of the quadriceps muscle and, in particular, the VMO has been demonstrated in patients with PFP,[5,13,19,22,32] it seems valuable to investigate if muscle size has any influence on the functional outcome after treatment. Moreover, because maximum force-generating capacity (strength) cannot be measured accurately and separately in muscles such as VMO, there is potential to use change in muscle size as an indicator of muscle strengthening and hypertrophy.[6]

The authors who included knee extensor muscle strength in their analyses have only measured strength isometrically or concentrically.[28,33,39] However, it is generally accepted that patients with PFP usually have slight concentric strength deficits, whereas their eccentric strength is remarkably lower compared with healthy persons.[3,34,37] Consequently, it might be useful to evaluate the extent to which eccentric knee extensor strength at baseline plays a role in the outcome after treatment. To the best of our knowledge, this has never been studied.

More insight is required to identify factors that could predict the outcome of a conservative rehabilitation program. Therefore, this study uses a prospective study design to identify factors that could predict the short-term functional outcome and account for the variation frequently seen in the outcome after conservative treatment of PFP.

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