Normal range of motion is a key to patient satisfaction. 

A normal arc of motion is the result of many of the technical considerations discussed in former sections.  It depends on matching flexion and extension gaps, normal condylar offset, and restoration of the joint line.  Lets look at each of these variables.

1) Gap balancing.   While loose gaps create instability, a tight gap limits motion.  A tight flexion gap prevents flexion.  When trialing a tight flexion gap, the poly will lift off the tray or get spit out.  A tight extension gap limits full extension, and when trialing a tight extension gap, the knee will not fully extend during a straight leg rise. 

Remember the general rule that the tibia is responsible when the knee is tight in both flexion + extension, while the distal femur is responsible for extension only tightness, and the posterior femur is responsible for flexion only tightness.

2) Posterior condylar offset.  If you think about what causes a firm endpoint in knee flexion, its the posterior tibia impinging on the femur at roughly 140° flexion (in obese people, this endpoint can occur earlier in the arc of motion due to entrapment of fat rolls).  The posterior condyle acts to maximize distance before impingement (similar to a larger femoral head when discussing hip motion), therefore recreating the normal posterior condyle size with the femoral implant is critical to maximize distance before impingement.  Posterior condylar offset is determined by the AP measurement of the femur (which is the size of the femur).  Femur size relates to the AP diameter.  Under-sizing a femur (using an anterior referencing system) will decrease the posterior condylar offset and decrease the arc of motion because there is less offset between the posterior tibia and the posterior femur. A posterior referencing system will guarantee normal offset, however, you can still under-size the femur and cause notching of the anterior cortex.

3) Joint Line.  Recreating a normal joint line also affects motion because it changes the position of the patella during motion and affects the extensor mechanisms mechanical advantage.  When recreating the joint line during TKA, the patella is constant (doesn’t move) but its relationship to the joint line changes. For example, if you remove 12 mm of distal femur (remember all implants replace 9 mm of distal femur), then you’ve moved the joint line up by 3 mm, so the patella is now 3 mm lower in relation to the raised joint line.  This is called “Patella Baja”.  In contrast, if the joint line is lowered, the patella is now relatively higher and so its called “Patella Alta”.  In general its difficult to lower the joint line (creating the relative “Patella Alta”) because the poly inserts come in so many sizes (at 2 mm intervals) so if you accidentally buzz two millimeters extra off the tibia, you will go up 2 mm on the poly, and recreated the native joint line.  In comparison, the femoral component is 9 mm always, so taking 12 mm will raise the joint line (creating the Patella Baja), and while metal augments can restore the joint line during a revision case, there are far viewer options (augments come as 5 mm, 10 mm etc). 

There are radiographic measures to determine the position of the joint line.  The Insall-Salvati Ratio can describe the relative position of the patella by measuring the distance between the distal pole of the patella to the proximal tibia divided by the length of the patella itself.  The denominator (the patellar length) is constant.  The distance of the patella from the tibia (the patellar tendon length) is variable.  If patella is sitting too close to the tibia, its sitting too low, and therefore its Baja.  There are other generalizable radiographic criteria.  The Joint line is 10 mm above the fibular head, 25 mm below the lateral epicondyle, and 35 mm below the medial epicondyle.

To understand why Patella Baja alters range of motion, we must first understand how the patella functions overall.  The patella increases the leverage of the quad muscle during leg extension.  

The joint line is determined by the distal femur cut, not the tibial cut.  Why?  Because every femoral size has the same 9 mm of distal femur offset, while the tibia has many sizes of polyethylene.  If you cut the tibia too low, say cut 12 mm instead of the desired 9 mm, you can build it back up by adding a 12 mm poly instead of the 9 mm poly you initially planned for. 

In contrast, if you accidentally place your distal femoral cutting jig to take off 11 mm, you cannot compensate for that extra 2 mm because all sizes of the femoral component is 9 mm of distal femur.  Therefore you have taken off 11 mm of bone, and only put back 9 mm of metal, effectively raising the joint line by 2 mm.  Now the joint line is sitting higher in relation to the patella (or another way of saying this is that the patella is sitting lower in relation to the joint line, aka a “patella baja”).  The degree of Patella Baja is measured by the Insall-Salvati ratio which compares the length of the patella (a constant and thus the denominator) with the distance between the tibial tubercle and the inferior pole of the patella. 

Patella Baja is rarely an issue in primary TKA.  The rare circumstance occurs when the patient has previously had a tibial tubercle osteotomy, or trauma to the patellar tendon that leads to scarring and a pre-operative patella baja.  In contrast, restoring the normal joint line is an important consideration during revision surgery, when metaphyseal bone is lost after explanting the femoral component.  There are some landmarks to identify the native joint line in cases of significant bone loss.  The joint line is approximately 10 mm above the fibular head, 25 mm below the lateral epicondyle, 35 mm below the medial epicondyle, and with the leg in extension the joint line should typically sit at the inferior pole of the patella.  In cases of revision surgery, distal femoral augments can be added to lower the joint line to the appropriate position (again its almost never an issue raising the joint line).  Remember that adding distal femoral augments will close the extension gap without affecting the flexion gap.