Influence of Knee Abductor Moment on Patellofemoral Joint Stress and Self-reported Pain of Women with Patellofemoral Pain

Abstract

Increased knee abductor moment (KAM) is thought to be related with elevated patellofemoral joint (PFJ) stress, a major contributor to patellofemoral pain (PFP). Knowing which parameter of the KAM relates with PFJ stress and self-reported pain is important as interventions can vary depending on the altered KAM parameter. This study aimed to compare peak, rate of moment development and impulse of KAM and PFJ stress of women with PFP and pain-free controls during stair descent; and to investigate the relationship among these variables with self-reported pain. Sixty-four women aged 18–35 years were recruited. A three-dimensional motion analysis system with link-segment models and inverse-dynamics equations was used to obtain kinetic data during a stair descent task. A previously reported algorithmic model was used to determine patellofemoral contact force (PCF) and patellofemoral contact pressure (PCP), indicatives of PFJ stress. Self-reported pain was assessed using a visual analogue scale (VAS). Women with PFP presented higher peak, rate of moment development and impulse of the KAM, PCF and PCP during stair descent than pain-free controls, suggesting women with PFP experience higher levels of PFJ stress while descending stairs. Only KAM impulse presented positive moderate correlations with self-reported pain, PCF and PCP. These findings indicate that strategies aimed at decreasing KAM impulse could reduce the load over the PFJ and improve pain of women with PFP.

Appendix: Algorithm Description for Calculation Patellofemoral Joint Contact Force and Pressure

Patellofemoral joint contact force during stair descent was estimated as a function of knee flexion angle (fa) and knee extensor moment (Em) according to the biomechanical model described by [20]. Firstly, an effective moment arm of the quadriceps muscle (mq) was calculated as a function of knee flexion angle based on cadaveric data presented by [30]:

$$ {\text{mq}} = 0.00008{\text{fa}}^{ 3} {-}0.013{\text{fa}}^{ 2} + 0.28{\text{fa}} + 0.046 $$

(1)

Quadriceps force (QF) was then calculated using the following formula:

$$ {\text{QF}} = {\text{Em}}/{\text{mq}} $$

(2)

PCF was estimated using the QF and a constant (K):

$$ {\text{PCF}} = {\text{QF}}\,\,{\text{K}} $$

(3)

The constant was calculated in relation to the fa using a curve fitting technique described by van Eijden et al. [34]:

$$ \begin{aligned} {\text{K}} = & \left( {0.462 + 0.00147{\text{fa}}^{ 2} {-}0.0000384{\text{fa}}^{ 2} } \right)/ \\ & \,\left( {1{-}0.0162{\text{fa}} + 0.000155{\text{fa}}^{ 2} {-}0.000000698{\text{fa}}^{ 3} } \right) \\ \end{aligned} $$

(4)

PCP (MPa) was calculated as a function of the PCF divided by the patellofemoral contact area. The contact area was described in accordance with Powers et al. [35] who documented patellofemoral contact areas at varying levels of knee flexion (83 mm² at 0°, 140 mm² at 15°, 227 mm² at 30°, 236 mm² at 45°, 235 mm² at 60°, and 211 mm² at 75° of knee flexion).

$$ {\text{PCP}} = {\text{PCF}}/{\text{contact}}\,{\text{area}} $$

(5)