Research Review Corner: June 2009

Patellofemoral joint (PFJ) pain is a common and often perplexing
clinical condition that is still poorly understood.  Recently it has
been suggested that PFJ stress can result from improper mechanics or
motor control of proximal structures.

Study Title: Differences in hip kinematics, muscle strength, and muscle activation between subjects with and without patellofemoral pain
Authors: Souza RB, Powers CM
Publication Information: Journal of Orthopaedic & Sports Physical Therapy 2009; 39(1): 12-19.

Background Information:
Patellofemoral joint (PFJ) pain is a common and often perplexing clinical condition that is still poorly understood.  Recently it has been suggested that PFJ stress can result from improper mechanics or motor control of proximal structures.  More specifically, altered kinematics of the femur and hip joint have been implicated in abnormal patellar tracking and pain syndromes in the anterior knee^1-3.

One of the most prevalent mechanisms thought to contribute to PFJ pain is increased femoral (and hip) internal rotation, both static and dynamic. Various researchers have suggested that structural internal rotation of the femur with diminished dynamic control of internal rotation during weight-bearing activities is a possible injury mechanism in PFJ pain.  With increased internal rotation, PFJ stress increases1 and a dynamic valgus of the lower extremity occurs, resulting in higher forces acting laterally on the patella.  Proper hip muscle activation, strength, and timing, particularly in the gluteus medius and maximus, could combat these mechanisms.

Based on these concepts, the aim of this study was to determine whether females with PFJ pain demonstrate different hip mechanics and muscle strength measures compared to pain-free controls.  

Pertinent Results:

• When averaged across all tasks, the PFJ pain group demonstrated increased peak hip internal rotation during dynamic tasks (p < 0.001).
• The largest difference in peak internal rotation between groups was observed during the running task.
• No significant group effect was observed for hip adduction (this is contrary to other literature in this area – see below).
• PFJ pain subjects demonstrated lower strength in both isometric measurements – hip extension and hip abduction – compared to controls (p = 0.02 and 0.005 respectively).
• EMG recordings revealed increased gluteus maximus (GM) activation in the PFJ pain group during the step-down (64 per cent greater) and running (91 per cent greater) tasks compared to controls (p = 0.041). This suggests that these patients are attempting to recruit a weakened muscle, perhaps to control hip rotation.
• The fact that EMG activity of the GM was not greater during the drop jump task may be explained by the fact that this was a bilateral task (previous literature has demonstrated a difference during similar unilateral tasks).
• EMG recordings were not significantly different between groups for the gluteus medius.

Conclusions and Practical Application:
This study corroborates findings from previous studies 2,3 which, taken as a whole, suggest that young female subjects with PFJ pain have altered hip kinematics and function compared to pain-free controls.  Readers should remember that most studies on this condition are performed on females, as they have a much higher prevalence of this condition compared to males.

Previous studies suggest that patients with PFJ pain demonstrate altered lower extremity kinematics and hip muscle strength – specifically greater hip adduction during running, single-leg squatting, and repetitive single leg jumping tasks; as well as reduced hip muscle strength when compared to pain-free individuals.  In this study, PFJ patients also had diminished hip muscle strength, as well as greater peak hip internal rotation and altered muscle recruitment strategies.  It should be noted that the cross-sectional nature of this study can only suggest an association between hip function and PFJ pain, but prevents any establishment of a cause and effect relationship.

From a treatment perspective, the increased internal hip rotation and reduced hip extension strength would point astute clinicians toward a treatment program that assesses soft tissue tension in the hip area and normalizes hip joint/pelvic motion.  Further, it may be prudent to implement a rehabilitation program that improves GM and medius strength and function.  The GM is a primary hip extensor, as well as an eccentric controller of hip internal rotation during dynamic tasks.  The gluteus medius is a lateral hip stabilizer that may prevent excessive dynamic hip adduction.  Further research is required to establish the most effective approach however.

Study Methods:
Two groups were used in this study: 21 females with PFJ pain and a comparison group of 20 age-matched, pain-free controls.  Subjects were between the ages of 18-45 (older subjects could potentially have degenerative joint disease).  Subjects in the study group had PFJ pain that was assessed by a physical therapist to first rule out ligamentous instability, internal derangement, patellar tendinopathy, or intra-articular effusion.  To participate in the study, subjects had to have:

• pain located around the PFJ
• reproducible pain with at least two of the following: stair descent, kneeling, prolonged sitting, or isometric quadriceps contraction
• duration of pain, greater than three months

Those with a previous history of knee surgery, traumatic patellar dislocation, or neurological deficit that would influence gait were excluded.

Subjects attended two laboratory sessions, and all subjects were issued a standard pair of running shoes to minimize that potential confounder.  For patients with bilateral symptoms, the more painful side was used for testing.
On the first visit, three dimensional motion analysis and EMG recordings were taken during three tasks:

1. running
2. step-down
3. drop jump

At the second lab visit, isometric strength measurements were recorded for the following motions using a handheld dynamometer:

1. hip extension (performed prone with knee bent)
2. hip abduction (performed side-lying)

Group differences were assessed across tasks using a mixed-design two-way analysis of variance and independent t-tests.

References

1. Lee TQ, Morris G, Csintalan RP. The influence of tibial and femoral rotation on patellofemoral contact area and pressure. JOSPT 2003; 33: 686-693.
2. Willson JD, Davis IS. Lower extremity mechanics of females with or without patellofemoral pain across activities with greater task demands. Clin Biomech 2008; 23: 203-211.
3. Bolga LA et al. Hip strength and hip and knee kinematics during stair descent in females with and without patellofemoral pain syndrome. JOSPT 2008; 38: 12-18.

Dr. Shawn Thistle is the founder and president of Research Review
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