Why Do I Have Knee Pain? Complete Guide to Patellofemoral Pain Syndrome: Part 2 – Biomechanics of the Knee

By djpope

March 25, 2018

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Patellofemoral pain syndrome (PFPS) is the most common form of knee pain treated in physical therapy clinics.  I made this series to help people better understand the condition, why it occurs and how to fix it.  So in part 1 we went over the prevalence, presentation and anatomy associated with patellofemoral pain syndrome.  If you missed that part I’d definitely recommend going back and checking it out.

In part 2 we’ll be discussing the biomechanics of the knee as it relates to PFPS.  Before we get into the specifics, there are 2 major biomechanical factors to understand that occur during PFPS:

  1. Patellofemoral Stress
  2. Patellar Tracking

1: Patellofemoral Stress 

When we perform movements like squatting, running and stairs the patellofemoral (PF) joint takes stress.   It’s important to understand this concept.  No matter how perfect our technique is or how strong we are the PF joint will take stress during these movements.  Also keep in mind that this joint was made to handle this stress and has very durable cartilage to help handle the forces of your activities.

However, sometimes when there is too much stress knees can become painful.  This is a central concept of PFPS.  Also, when recovering from PFPS we want to ensure we don’t overly stress the joint but apply the correct dosage of stress to recover.  Therefore it makes sense that we should have some sort of awareness of what activities and motions create more or less knee stress.  Lucky for us, the amount of stress in the PF joint can be measured so we can get an idea of just how much stress the joint takes and which movements create the most (or least) stress to the joint.

There are 2 things to understand when discussing stress in the PF joint.  The first is the amount of total surface area available in the joint to dissipate force.  Before we get into this subject I wanted to first have a little discussion on surface area and stress.

A central concept to understand is that a larger surface area dissipates force more easily then a smaller surface area.  In other words, the same force applied to a large surface area is less force per square inch then that same force to a smaller surface area.

Here’s another way to think about it.  Let’s compare being punched by a fist vs. being punched with a knife.  The surface area of the fist is broad and when it hits your shoulder it may hurt but probably won’t cause much damage.  Let’s contrast that with the knife.  If the knife hits your shoulder with the same force, the surface area of the pointy knife is so small it would go right through your shoulder, causing quite a bit of damage.

Small surface area (knife) = hard to dissipate force

Broad surface area (fist) = easier to dissipate force

Based on this rule, having a large surface area to dissipate force in the knee is good.  However, the amount of surface area available between the patella and femur to dissipate force during movement is variable depending on how much the knee is bent.

When the knee is fully straightened there is actually no contact (surface area available) of the femur against the patella.  When we first start to bend our knees, in the first 20 degrees of motion the patella begins to engage the femur.  Initially there is very little boney contact between the two structures.  However, as the knee starts bending further there is more and more contact (surface area) between the patella and femur.  The surface area of contact continues increasing until about 90-120 degrees of knee flexion.  At this point the surface area actually begins decreasing as we approach full knee flexion (bending) (2).

So this point is fairly complex to understand so we’ll re-cap below (2):

  • 0 degrees knee flexion = no contact between patella and femur
  • 0-90 degrees of knee flexion = surface area contact steadily increases (**Wrapping effect begins at 70 degrees)
  • ~90 degrees knee flexion = maximal surface area contact between patella and femur
  • 90-135 degrees knee flexion = surface area contact decreases
  • >135 degrees = surface area contact continues to decrease and is lowest at end range knee flexion

**Side Note: To make matters a bit more confusing, as the knee flexes past 70 degrees the quadriceps tendon comes into contact with the femur.  This also serves to increase surface area contact within the patellofemoral joint potentially increasing surface area available to dissipate force.  This is known as the “wrapping effect”.  This wrapping effect may potentially help to dissipate forces as we descend to depths below 70 degrees, however there is not much research to either support or refute this claim.

So here’s the thing.  Increased surface area between the patella and the femur is a good thing.  More contact gives more area to dissipate forces within the joint.  Therefore, around 90 degrees of knee flexion there should be the least amount of stress through the knee joint right?  We should all perform our squats to 90 degrees of knee flexion to minimize stress on the knee?

I wish it were that simple…

The second principle to understand are joint reaction forces and quad contraction.  Basically the more the knee goes into flexion (knee bending) and the harder the quadriceps contracts, the greater the forces on the patellofemoral joint.

In more simplistic terms, as we descend deeper into a squat the patellofemoral forces steadily increase.

So, despite the surface area of the joint increasing as we descend to 90 degrees of knee flexion, the PFJ forces increase substantially and are not off-set by the increased surface area of the joint (2).  

Food for thought: From an evolutionary perspective maybe our knees developed more surface area around 90 degrees of knee flexion in an attempt to buffer some of the increased stress that comes from loaded knee flexion.

Activities that take us into more and more knee bending tend to stress the knee more and more.  We have some research to show how much stress goes through the PF joint during several different activities (4).  Notice that activities that require more knee bending tend to place larger stresses through the PF joint:

  • 1.3 times body weight during level ambulation
  • 3.3 times body weight during stair ambulation
  • 5.6 times bodyweight during running
  • 7.8 times bodyweight during a deep knee bend or squat

Now PFPS is largely thought to occur from overuse of the PF joint.  From this graph it can be pretty easy to see why running and squatting can create knee pain if abused and why stairs can be irritating once PFPS has started.  This also helps to explain why those who perform a lot of deep squatting are susceptible to knee pain.

Decreased surface area contact at bottom of squat + high quadriceps contraction = Lots of PF joint stress

I’m not saying to stop squatting but I am saying it makes a lot of sense why these athletes get painful knees from time to time, especially given that adding load to the squat also increases stress on the PF joint (16).

Side Note: For this discussion we’re describing how the joint takes forces during closed chain activities like squatting, running and stairs.  These principles are not the same when we stress the joint in an open chain fashion (when the knee is fixed and the foot swings freely.  Think of a seated knee extension).  Lucky for us, most of the movements we perform in life and in the gym that load the PF joint are performed in this closed chain fashion.  

Another factor that can increase stress in the PF joint is tightness of the quad musculature (1).  This is documented as a patellofemoral compressive issue (1).  The quadriceps muscle attaches directly to the knee cap and then the knee cap attaches directly to the tibia.  This forms a strap.  As the knee flexes further and further the strap tightens and compresses the patella into the trochlear groove causing compression within the PF joint.  This is potentially the reason why patients with PFPS also have pain with prolonged sitting. They’re just sitting in a position which compresses the joint for a long period of time and can eventually cause pain.

Patients with PFPS have also been shown to present with increased tightness of their quads and hip flexors (through thomas testing), as well as tightness in the IT band (Ober testing) (1).  Restoring normal flexibility (Thomas and Ober tests) in those areas has been shown to be a predictor of recovery from PFPS (1).

2: Patellar Tracking

So in part 1 of this article series we discussed the trochlear groove of the femur and how the patella fits into this groove.  As the knee bends the patella tracks down along this groove in the femur.  Like we spoke about earlier, having optimal alignment of the patella in the groove is important for the health of the joint.  This alignment allows greater surface area of the joint to dissipate forces and ensures the forces are spread evenly throughout the joint.  Several factors can effect this tracking and we’ll discuss these below.

Remember that in part 1 of this series we spoke about the active and passive stabilizers of the PF joint.  One of the passive stabilizers of the knee was the boney fit of the patella into the trochlear groove.  The better the fit, the easier tracking is.

It’s important to understand that this fit is variable and not as good from person to person (4).  Unfortunately this is not something that we can fix with surgery and these folks sometimes have trouble with dislocation of the knee cap (which is a topic for another discussion).  Check out these different variations of trochlear grooves below:

People with a hypermobile patella have been shown to be at increased risk of developing PFPS.

In PFPS there is generally increased stress to the lateral (or outside) portion of the PF joint.  This will occur for a variety of reasons but one potential cause for this can be tightness on the lateral (outside) portion of the leg.  These potentially tight structures include:

  • Lateral patellofemoral ligament
  • Iliotibial band (IT Band)
  • Lateral retinaculum

Tightness of the IT band pulls directly on the patella through the lateral retinaculum.  This is theorized to pull the patella laterally (to the outside of the body) and cause mal-tracking.  This increases stress to the lateral side of the joint (1).  Fortunately as we’ll discuss later, normalizing this flexibility has been shown as a predictor of those who rehabilitate from PFPS (1).

Keep in mind that it isn’t just tightness on the outside of the leg that can produce issues.  We could also have looseness, weakness and or timing issues of the medial stabilizers.  Looseness or weakness of these structures can cause the knee cap to track laterally in it’s groove during exercise.  Some of these structures are the:

  • Joint Capsule
  • Medial meniscopatellar ligament
  • Medial retinaculum
  • Medial patellofemoral ligament
  • Quadriceps (VMO)
  • Pes anserine musculature
  • Biceps femoris

Remember that the vastus medialis oblique is an active medial stabilizer of the knee.  Individuals with PFPS tend to have delayed firing of the VMO.  A lack of control from this muscle which could alter patellar tracking (8).  We also have some research to show that altered firing times of the VMO can predict people who go on to develop PFPS. 

So to recap:

  • Patellofemoral pain syndrome is largely a condition of overuse
  • Poor patellar tracking can increase stress within the PF joint
  • Activities with increased knee bending increase stress to the PF joint
  • Increased load and quadriceps contraction increases stress within the PF joint
  • Looseness or weakness of structures on the medial (inside) side of the knee joint can affect patellar tracking
  • Tightness of structures on the lateral (side) or anterior (front) of the knee joint can increase compressive stress in the PF joint

Now you know that PFPS is an overuse condition and you also know why certain activities place more stress on the PF joint then others.  Lastly you know that patellar tracking can affect PF stress and how certain structures around the knee can affect this.  In the next article we’ll be discussing how the foot, ankle and hips can also play a role in this situation.  I can’t wait…

Click HERE for Part 3:

Who Doesn’t Love a Little Knee Pain?

Dan Pope DPT, OCS, CSCS, CF L1

Works Cited:

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