Orthopedics

Seven Cuts to the Perfect Total Knee

Peter Brooks, MD, FRCS(C)

Abstract

There are a total of 7 bone cuts in a typical total knee replacement (TKR): distal femur, anterior femur, posterior femur, anterior chamfer, posterior chamfer, tibia, and patella. Each of these cuts has its own special science, and each cut can affect the other cuts and potentially the outcome of the TKR.

The distal femoral cut starts the overall alignment of the leg. Five degrees of valgus is cosmetically appealing, avoids excessive valgus, and prevents thighs from rubbing together. The anterior femoral cut sets femoral component rotation, which has effects on patellar tracking and gap balancing. In most knees, correct rotation is approximately 3° of external rotation compared to the posterior condylar axis. An important exception is in valgus knees, where this could lead to accidental internal rotation. The posterior condyle cuts, with the tibial cut, determine the flexion gap. Injury to the medial collateral and posterior cruciate ligaments should be avoided. Anterior and posterior chamfer cuts must avoid these ligaments as well. The tibial cut is challenging. A 3° posterior slope is most typical, and rotation is crucial. Internal rotation is a common error, affecting patellar tracking. Changing rotation on a sloped cut also adds varus or valgus. The patella cut should not be too deep. Component placement should tend medial and superior. If a lateral release is necessary, it should be done from inside-out, with preservation of the blood supply.

There are a total of 7 bone cuts in a typical total knee replacement (TKR): distal femur, anterior femur, posterior femur, anterior chamfer, posterior chamfer, tibia, and patella. Each of these cuts has its own special science, and each cut can affect the other cuts and potentially the outcome of the TKR.

I prefer to make this cut at 5° to the intramedullary axis, using a cannulated guide rod to help prevent fat embolism. Five degrees valgus is cosmetically appealing, avoids accidental excessive valgus, and prevents obese thighs from rubbing together. It is important not to push the intramedullary guide too far in, as the femoral bow will add some extension to the distal cutting block and may lead to notching and fracture. The depth of the distal femoral cut (with the tibial cut) sets the extension gap, and additional bone may be resected to correct a flexion contracture, usually no more than 2 mm. Rotation does not matter yet.

I use posterior referencing, so I make the anterior femoral cut next. This sets both the size and the rotation of the femoral component. Typically, the rotation is set at 3° external to the posterior condylar axis. An exception to this rule is the valgus knee, where the lateral femoral condyle may be dysplastic. In these cases, reliance on the posterior condylar axis will lead to internal rotation and patellar maltracking. A better guide is the epicondylar axis and Whiteside’s line (Figures 1, 2). A good anterior cut looks like a grand piano.

Figure 1: The epicondylar axis (on which the femoral component is set) is 3° externally rotated to the posterior condylar axis. Whiteside’s line, from the bottom of the trochlear groove to the top of the intercondylar notch, is at right angles to the epicondylar axis. Figure 2: In the valgus knee, the posterior condylar axis is distorted by the dysplastic lateral femoral condyle, but the relationship between the epicondylar axis and Whiteside’s line remains constant.

The posterior femur cut, with the tibial cut, sets the flexion gap. A common error is to injure the posterior cruciate ligament (PCL) or the medial collateral ligament (MCL), so I prefer to use a narrow saw blade for this…

Abstract

There are a total of 7 bone cuts in a typical total knee replacement (TKR): distal femur, anterior femur, posterior femur, anterior chamfer, posterior chamfer, tibia, and patella. Each of these cuts has its own special science, and each cut can affect the other cuts and potentially the outcome of the TKR.

The distal femoral cut starts the overall alignment of the leg. Five degrees of valgus is cosmetically appealing, avoids excessive valgus, and prevents thighs from rubbing together. The anterior femoral cut sets femoral component rotation, which has effects on patellar tracking and gap balancing. In most knees, correct rotation is approximately 3° of external rotation compared to the posterior condylar axis. An important exception is in valgus knees, where this could lead to accidental internal rotation. The posterior condyle cuts, with the tibial cut, determine the flexion gap. Injury to the medial collateral and posterior cruciate ligaments should be avoided. Anterior and posterior chamfer cuts must avoid these ligaments as well. The tibial cut is challenging. A 3° posterior slope is most typical, and rotation is crucial. Internal rotation is a common error, affecting patellar tracking. Changing rotation on a sloped cut also adds varus or valgus. The patella cut should not be too deep. Component placement should tend medial and superior. If a lateral release is necessary, it should be done from inside-out, with preservation of the blood supply.

There are a total of 7 bone cuts in a typical total knee replacement (TKR): distal femur, anterior femur, posterior femur, anterior chamfer, posterior chamfer, tibia, and patella. Each of these cuts has its own special science, and each cut can affect the other cuts and potentially the outcome of the TKR.

1) Distal Femur Cut

I prefer to make this cut at 5° to the intramedullary axis, using a cannulated guide rod to help prevent fat embolism. Five degrees valgus is cosmetically appealing, avoids accidental excessive valgus, and prevents obese thighs from rubbing together. It is important not to push the intramedullary guide too far in, as the femoral bow will add some extension to the distal cutting block and may lead to notching and fracture. The depth of the distal femoral cut (with the tibial cut) sets the extension gap, and additional bone may be resected to correct a flexion contracture, usually no more than 2 mm. Rotation does not matter yet.

2) Anterior Femur Cut

I use posterior referencing, so I make the anterior femoral cut next. This sets both the size and the rotation of the femoral component. Typically, the rotation is set at 3° external to the posterior condylar axis. An exception to this rule is the valgus knee, where the lateral femoral condyle may be dysplastic. In these cases, reliance on the posterior condylar axis will lead to internal rotation and patellar maltracking. A better guide is the epicondylar axis and Whiteside’s line (Figures 1, 2). A good anterior cut looks like a grand piano.

Figure 1: The epicondylar axis is 3° externally rotated to the posterior condylar axis Figure 2: In the valgus knee, the posterior condylar axis is distorted

Figure 1: The epicondylar axis (on which the femoral component is set) is 3° externally rotated to the posterior condylar axis. Whiteside’s line, from the bottom of the trochlear groove to the top of the intercondylar notch, is at right angles to the epicondylar axis. Figure 2: In the valgus knee, the posterior condylar axis is distorted by the dysplastic lateral femoral condyle, but the relationship between the epicondylar axis and Whiteside’s line remains constant.

3) Posterior Femur Cut

The posterior femur cut, with the tibial cut, sets the flexion gap. A common error is to injure the posterior cruciate ligament (PCL) or the medial collateral ligament (MCL), so I prefer to use a narrow saw blade for this cut. The assistant’s medial retractor should be raised up parallel to the saw-cut to protect the MCL.

4 & 5) Anterior & Posterior Chamfer Cuts

These are simple cuts usually made through the same cutting block as the anterior and posterior femoral cuts. Again, take care not to injure the MCL and PCL.

6) Tibial Cut

I prefer a 3° posterior slope. I only use a 0° slope when planning a stemmed tibia. I use a combination of intra- and extramedullary guides. The intramedullary guide is best for the 3° posterior slope, and the extramedullary guide is best for setting a neutral varus–valgus position. Rotation of the tibial cut is critical. Excessive internal rotation is one of the most common errors of tibial component position and leads to patellar maltracking. Also, once a sloped cut is made, further changes to component rotation add varus or valgus to the overall alignment.

One way to judge rotation of the jigs, blocks, and tibial component is by the tibial tubercle. A straight finger passed along the handle of the cutting guide should just brush against the medial edge of the tubercle (Figure 3). This places the axis of the component at the junction of the medial and middle thirds of the tubercle. At the same time, the extramedullary guide should be bisecting the ankle. Finally, when seen from above, the posteromedial tibia should be uncovered while the posterolateral tibia is completely covered (Figure 4). This is because the medial tibial plateau is anatomically longer than the lateral, so a symmetrical baseplate cannot cover both equally.

Figure 3: The intramedullary guide sets the posterior slope Figure 4: Correct rotation of the tibial baseplate leaves a little uncovered bone
Figure 5: The lateral retinaculum can then be released deep to the vessels

Figure 3: A straight finger just touching the tubercle assists in correct rotation of the tibial jig in this right knee. The intramedullary guide sets the posterior slope, while the extramedullary guide bisects the ankle in proper rotation and ensures a neutral varus–valgus cut. Figure 4: Viewed from above, correct rotation of the tibial baseplate leaves a little uncovered bone on the posteromedial tibia. Figure 5: In this right knee, the foot is to the right, the head to the left. The superior lateral geniculate vessels, proximal to the superior pole of the patella, are seen just as the synovium is divided. The lateral retinaculum can then be released deep to the vessels.

7) Patellar Cut

Most surgeons in the United States resurface the patella, while many in Europe and most in the Middle East do not. No universal doctrine is therefore applicable, but my preference is to resurface. Large central pegs are associated with patellar fracture, and metal-backed patellae have excessive wear, so I use an all-poly patella with 3 pegs. Anatomic or offset dome designs more closely match the anatomy than simple domes. Avoid distal placement of the patellar button, which can lead to impingement on the tibial insert, and avoid lateral placement, which makes tracking difficult. Place the button medial and superior. If a lateral retinacular release is necessary, I prefer to do these from inside-out, and I identify and preserve the superior lateral geniculate vessels that appear above the superior pole, covered just by the synovium (Figure 5).

Author

Dr Brooks is from the Cleveland Clinic, Cleveland, Ohio.

Dr Brooks is a paid consultant for Stryker and Smith & Nephew.

Presented at Current Concepts in Joint Replacement 2008 Winter Meeting; December 10-13, 2008; Orlando, Florida.

Correspondence should be addressed to: Peter Brooks, MD, FRCS(C), Cleveland Clinic, 9500 Euclid Ave A41, Cleveland, OH 44195.

DOI: 10.3928/01477447-20090728-27

10.3928/01477447-20090728-27

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