RECONSTRUCTION 2023, Jan, 08 | 12:00 am

Posterior-Stabilized Technique

P ITFALLS

  • PS total knee replacement is not used when there is deficient collateral ligamentous support— it provides no medial lateral constraint.

     

  • A more constrained prosthesis is needed when balance in flexion and extension cannot be obtained.

 

Posterior Stabilized Technique

 

Indications

  • The indications for a posterior-stabilized (PS) total knee replacement are those for knee arthroplasty in general: end-stage degenerative changes in one or more compartments of the knee.

  • While some may reserve a PS technique for complicated deformities only, it has demonstrated excellent results in all patients, and better results in patients following patellectomy.

    Examination/Imaging

    Controversies

    • The debate between cruciate-retaining and cruciate-substituting knee arthroplasty continues. Both have shown good results. Concerns of PS are those of increased bony resection, and potential for tibial post wear.

     

    Physical Exam

  • Examination of the knee includes evaluation of the hip, overall limb alignment, and vascularity. It is essential to be sure there is no hip pathology that needs to be addressed first.

  • Preoperative range of motion should be noted.

  • Evaluation of ligamentous stability is essential, with special attention to laxity in the varus/valgus plane and the ability to correct the deformity. This is helpful in understanding the releases that will be necessary, and it may also impact the amount of bony resection that is performed. This should be repeated under anesthesia.

    Treatment Options

    • Alternative treatments include nonoperative management. Partial knee arthroplasty may be considered for single-compartment arthritis, and osteotomy is an option for younger active patients with limited joint involvement.

     

    Plain Radiographs

  • Preoperative radiographs, including a standing anteroposterior view, are evaluated for overall alignment, extent of deformity, and bone loss.

  • Posteroanterior flexed views can be beneficial to show joint space narrowing, particularly with involvement of posterior condyles as with a valgus deformity.

  • Radiographs are templated for size, bone resection thickness, and reconstruction of the mechanical axis (Fig. 1A and 1B).

  • It is helpful to outline osteophytes that will affect balance and should be removed.

    Surgical Anatomy

  • Multiple injuries can occur during a PS procedure, and thorough knowledge of the surgical anatomy will help to prevent these (Fig. 2).

  • The menisci and cruciate ligaments are removed, and the collateral ligaments need to be protected.

    • The medial collateral ligament can be injured during the tibial cut, during the posterior femoral cut, or during removal of the medial meniscus.

 

 

 

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Posterior Stabilized Technique

 

A B

FIGURE 1

 

 

 

Notching of anterior cortex

 

 

Injury to posterior neurovascular

structures

Damage to bony surface fracture of condyle

 

Injury to LCL and popliteal tendon

Fracture of

patella Injury to LCL

 

Injury to common   peroneal nerve

 

Injury to MCL

 

Damage to tibial cut surface

Avulsion of ligamentum patellae

 

FIGURE 2

 

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Posterior Stabilized Technique

 

  • The lateral collateral ligament can also be damaged during removal of the lateral meniscus.

    • The popliteus tendon can be injured with removal of the lateral meniscus.

    • The popliteal vessels and tibial nerve are at risk posteriorly during the tibial cut.

    • The common peroneal nerve can be injured with a misplaced retractor laterally as it comes around the fibular neck.

    • Aggressive retraction of the patella can lead to patellar tendon avulsion or deformity of the cut surface of the patella.

    • Hohmann retractors can damage the cut surface of the tibia.

    • Additional potential injuries include fracture of the patella, notching of the anterior cortex during the anterior cut, and fracture during impaction of the prosthesis.

       

      P EARLS

      • The leg holder should be positioned to allow for maximum flexion (see Fig. 3).

         

      • Metal leg holders need to be well padded.

         

      • Avoid bulky padding anteriorly so the bony prominence can be palpated.

         

        P ITFALLS

      • Large legs may rotate externally; this can be prevented with a bump under the pelvis.

         

      • Leg holders may make it difficult to assess the position of the tibia, particularly in the sagittal plane.

       

      Positioning

    • The patient is placed supine with a nonsterile tourniquet on the proximal thigh.

    • A leg holder or bumps can be used on the operative table (Fig. 3).

    • With less invasive techniques, it is essential that the arthroplasty can be performed in various degrees of flexion to allow for maximum utilization of the “mobile window.”

      Portals/Exposures

    • The standard exposure for a knee arthroplasty is performed in flexion.

    • In this instance, a medial parapatellar arthrotomy extending into the rectus tendon is utilized. A small cuff of medial tendon is left attached to the muscle (Fig. 4).

      Equipment

      • A leg positioner that can be locked in any position of flexion works best.

       

    • A medial subperiosteal dissection of the proximal tibia, including the medial meniscus, is performed. The extent of dissection is dictated by the amount of deformity to be corrected.

    • The fat pad below the patellar tendon is removed, the anterior cruciate ligament transected, and the anterior horn of the lateral meniscus released from the midline. At this point the menisci are not removed, but are retracted to enhance exposure.

       

       

       

      39

       

       

      P EARLS

      • It is helpful to mark the edges of the tendon to expedite the repair (see Fig. 4).

         

      • A 2-mm cuff of tendon should be left with the vastus medialis to enhance repair.

         

        P ITFALLS

      • Visualization of the lateral plateau is essential for proper component sizing and alignment (Fig. 5).

       

      Posterior Stabilized Technique

       

      FIGURE 3

       

       

       

      FIGURE 4

       

       

       

      FIGURE 5

       

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      Instrumentation

      • Bent Hohmann retractors are helpful to protect the collateral ligaments.

       

      Posterior Stabilized Technique

       

      They are removed in their entirety after the bony cuts.

    • The patella is subluxed laterally and retracted with the lateral meniscus, to expose the proximal tibia (see Fig. 5).

      Controversies

      • The midvastus, subvastus, and quadriceps-sparing approaches may cause less trauma to the extensor mechanism and expedite rehabilitation. They are described later.

       

      Procedure

      Step 1

    • The distal femur is cut first (Fig. 6). This aids with smaller incisions.

    • The femoral canal is drilled (Fig. 7), and the intramedullary alignment rod is placed at a valgus angle corresponding to the templated angle.

       

      P EARLS

      • Most systems use a combination of measured resection and gap methods.

         

      • Templating will preview the starting point for the intramedullary drill and the relative resections of the distal condyles.

         

        P ITFALLS

      • Sitting the distal femoral resection guide on osteophytes may result in a limited resection.

         

      • Remove posterior osteophytes and release the capsule before removing excess distal femur for a flexion contracture.

       

    • The starting point is typically anterior to the posterior cruciate ligament (PCL). Templating can be helpful to determine the entry point (see Fig. 1).

    • Confirmation of central canal placement can be done with a smaller diameter long rod, which should easily pass into the medullary canal. The canal is suctioned prior to being instrumented to avoid marrow embolism.

    • Rotation is set according to the trochlea and the epicondylar axis.

      Step 2

    • Next the tibia is cut. An offset extramedullary tibial guide is used (Fig. 8).

  • Rotation is set off the medial one third of the tibial tubercle.

  • Distally it is placed in the center of the ankle joint (closer to the medial malleolus) (Fig. 9).

 

Controversies

  • The order of bony cuts is not important. After the distal femur, we cut the tibia as it allows room for the posterior referencing guide, and can aid in setting femoral rotation.

 

 

FIGURE 6

 

 

 

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Posterior Stabilized Technique

 

FIGURE 7

 

 

 

 

P EARLS

  • The amount resected is at least that needed to accommodate the tibial component and polyethylene. If there is significant wear of the plateau, a resection just below the sclerotic bone may be appropriate.

 

FIGURE 8

 

  • The tibial crest can be palpated to assess alignment.

  • Slope is set by approximating the sagittal axis of the tibia with the extramedullary alignment jig. To match the native slope, an angel wing can be placed on the resection block.

  • Finally, the depth of the resection is determined and correlated with the templated radiographs.

     

     

     

     

     

     

    P ITFALLS

    • In valgus knees and patients with lax ligaments, an initial resection of 9–10 mm may result in a large gap. It is important to assess the collateral ligamentous support prior to and during the procedure.

     

    Step 3

    FIGURE 9

  • After the resection is performed, a spacer block is placed on the cut surface of the tibia to assess the cut and alignment (both varus/valgus and slope) (Fig. 10).

     

     

    Controversies

    • Intramedullary and extramedullary tibial alignment have comparable results. The former cannot be used with canal deformity or obstruction. Extramedullary alignment is used to avoid further marrow embolization and for less invasive approaches.

     

     

    FIGURE 10

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    Posterior Stabilized Technique

     

     

     

     

    43

     

    Posterior Stabilized Technique

     

    FIGURE 11

     

     

    P EARLS

    • Measuring the amount of bone resected can help determine whether further resection of the distal femur or proximal tibia should be performed, if the extension space is tight.

     

    • The knee is placed in extension with the block to ensure an adequate extension gap (Fig. 11).

    • The knee is balanced in extension (Fig. 12).

       

      Instrumentation/ Implantation

      • An uncaptured jig can be used, and the alignment and extension gap are checked with the jig in place. If there is a need to resect more tibia, the jig is dropped.

       

       

      FIGURE 12

       

       

       

      44

       

       

      P EARLS

      • Use multiple means to assess rotation of the component. To assess the gap, the ligaments need to be under stretch; this is quickly done by lifting the femur up and pulling traction on the tibia at 90°.

         

        P ITFALLS

      • Posterior referencing can cause internal rotation when there is posterior lateral condylar wear, as in a valgus deformity. Be cognizant of the degree and extent of posterior condylar wear.

       

      Posterior Stabilized Technique

       

      FIGURE 13

       

      Step 4

  • Next the rotation of the distal femur is set by anatomic landmarks: the transepicondylar axis and Whiteside’s line (Fig. 13).

    Instrumentation/ Implantation

    • Many posterior referencing systems allow for various degrees of external rotation to accommodate posterior lateral condylar wear.

     

  • A posterior referencing system is used to set rotation of the posterior condyle (Fig. 14A) and to size the femur. The rotation is then compared to the anatomic landmarks (Fig. 14B).

  • Finally, prior to resection the gap is compared to the cut surface of the tibia cut to ensure a rectangular gap (Fig. 15).

 

 

 

A B

FIGURE 14

 

 

 

Posterior Stabilized Technique

 

FIGURE 15

 

Controversies

  • Bony landmarks may be less reliable than gap methods for assessing rotation.

  • With posterior referencing systems, downsizing the component may cause notching.

  • With anterior referencing, downsizing will open the flexion space.

 

Step 5

  • The anterior (Fig. 16A) and posterior (Fig. 16B) resections are made.

     

     

     

     

    45

     

    A

     

     

     

    FIGURE 16 B

     

    46

     

     

    P EARLS

    • Further femoral resection can easily be accomplished to equalize the gaps. This is more difficult after the chamfer and notch cuts.

       

    • Lifting the femur with a bone hook helps identify the medial collateral ligament.

     

    Posterior Stabilized Technique

     

  • At this point, the remaining menisci (Fig. 17A) and posterior cruciates (Fig. 17B) are removed.

  • Posterior osteophytes are removed next (Fig. 17C), and the capsule released if there is a significant flexion contracture.

  • The knee is balanced in flexion (Fig. 18A) and in extension (Fig. 18B), and the gaps are compared. The spaces in flexion and extension are assessed with the size of block that fits, and the symmetry to varus and valgus test.

  • A slightly tighter flexion gap is tolerated.

     

    P ITFALLS

    • It is imperative that no fibers of the PCL be intact, as this will tend to tighten the flexion space.

       

    • Care should be taken to not injure the posterior neurovascular structures.

     

    Step 6

  • The femoral finishing block (Fig. 19A) is used to cut the chamfers and the box.

  • The medial-lateral position of this block will determine position of the femoral component (Fig. 19B). Lateral placement will help enhance patellar tracking. The location of the intercondylar notch is also marked (Fig. 19C).

 

 

 

A B

 

 

 

C FIGURE 17

 

 

 

 

 

 

 

Posterior Stabilized Technique

 

 

 

 

 

 

 

47

 

A

FIGURE 18

 

B

 

A

 

B

 

C

 

FIGURE 19

 

 

 

P EARLS

  • Lines can be drawn in the box cut prior to fixing the block in place to be certain of the medial-lateral placement.

 

 

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P ITFALLS

  • It is important not to overcut the box or make any stress risers that may lead to a condylar fracture.

 

Posterior Stabilized Technique

 

FIGURE 20

Instrumentation/ Implantation

  • Various systems use osteotomes or chisels for the box cut.

 

  • If the femoral component needs to be downsized, a smaller block can be placed. In this case more bone will be resected posteriorly, opening the flexion space.

     

    P EARLS

    • A bone hook can be placed in the femoral trial component to ensure it does not go into flexion (Fig. 23).

       

      P ITFALLS

    • With smaller incisions, attention needs to be paid to the lateral placement of the tibial component as this can be difficult to visualize.

     

  • With the box and chamfer cutting guide in place (Fig. 20), resection is performed with a reciprocating saw (Fig. 21A and 21B).

    Step 7

  • The tibia is sized so there is no overhang of the component.

  • The tibial base plate is pinned into place and centered on the medial third of the tibial tubercle, and rotation is marked (Fig. 22).

  • The trial components are placed.

    Step 8

  • The patella is everted and cut freehand (Fig. 24). A towel clamp is used to hold the patella everted.

    • The patellar thickness is assessed before and after the resection.

    • When cutting freehand, two passes can be used.

       

      P ITFALLS

      • Over-resection of the patella may lead to fracture and under-resection to “overstuffing” of the patellofemoral joint.

       

      • The first pass starts medially and enters into the lateral facet. Superior-inferior thickness is assessed on the medial side.

        Controversies

        • We routinely resurface the patella. Outcomes of an un-resurfaced patella are likely related to implant design.

         

      • The second pass proceeds from the medial to the osteochondral junction laterally. With the second pass, compensations can be made for unequal superior-inferior resections, ensuring a uniform resection.

  • The patella is sized and medialized. The holes are drilled.

  • Any patellar bone that is uncovered and may articulate with the femoral component is marked and then bevelled with a saw or ronguer (Fig. 25).

 

 

 

Posterior Stabilized Technique

 

A B

FIGURE 21

 

 

 

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FIGURE 22 FIGURE 23

 

 

 

 

 

FIGURE 24 FIGURE 25

 

 

 

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Posterior Stabilized Technique

 

A

FIGURE 26

B

 

Step 9

 

P EARLS

  • If patellar tracking is an issue, external rotation of the tibia may enhance tracking. A release should not be performed until the knee is evaluated with the tourniquet deflated.

 

  • The knee is ranged.

  • Balance, range of motion, and patellar tracking are assessed.

  • The components are removed and, if patellar tracking was acceptable, the keel for the tibia is drilled and punched (Fig. 26A and 26B).

    Step 10

     

    P EARLS

    • Again a bone hook can be used carefully to inspect the flexion space (see Fig. 27).

       

      P ITFALLS

    • Small pieces of cement can be retained and lead to third body wear.

     

  • The tibia is cemented first, then the femur, and finally the patella (Fig. 27).

  • The cement gun is used to enhance penetration into the trabeculae.

  • The knee is held in extension as the cement hardens with a trial polyethylene liner.

  • The knee—particularly the posterior condylar region—is inspected for cement and the actual liner is inserted (Fig. 28).

    Step 11

  • The knee is closed in flexion (Fig. 29).

     

    P EARLS

    • It is important to stress to the patient and the therapist the importance of maintaining full extension.

       

    • Perioperative anesthesia and analgesia are important in expediting short-term recovery.

     

  • The ink marks are used to line up the rectus tendon.

    Postoperative Care and Expected Outcomes

  • Therapy begins the evening of surgery or the next morning.

  • A knee immobilizer is utilized when in bed to aid in full extension.

  • Continuous passive motion is not used.

 

 

 

 

 

Posterior Stabilized Technique

 

 

 

 

 

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FIGURE 27

 

FIGURE 28

 

FIGURE 29

 

 

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Posterior Stabilized Technique

 

Evidence

Fehring TK. Rotational malalignment of the femoral component in total knee arthroplasty. Clin Orthop Relat Res. 2000;(380):72-9.

 

The author compared two techniques of determining femoral rotation to balance the knee in flexion. The classic method of cutting the posterior condyles parallel to the cut tibial surface is compared to determine the rotation based on fixed femoral landmarks. Rotational errors of at least 3° occurred in 45% of patients when the rotation was determined from fixed femoral landmarks compared to the classic method.

 

Maestro A, Harwin SF, Sandoval MG, Vaquero DH, Murcia A. Influence of intramedullary versus extramedullary alignment guides on final total knee arthroplasty component position: a radiographic analysis. J Arthroplasty 1998;13:552-8.

 

The authors did a radiologic analysis of 116 consecutive cemented Kinemax cruciate-retaining knees, comparing the postoperative component. The coronal alignment of the tibial component with the intramedullary technique was found to be statistically significantly better (p  .01) than that with the extramedullary technique. Patients who were obese, with wide medullary canals, had less accurate alignment irrespective of technique.

 

Rasquinha VJ, Ranawat CS, Cervieri CL, Rodriguez JA. The press-fit condylar modular total knee system with a posterior cruciate-substituting design: a concise follow-up of a previous report. J Bone Joint Surg Am. 2006;88:1006-10.

 

The authors presented long-term results of primary PS modular knees in 118 consecutive patients. The authors had a good to excellent result in 90% of patients. The implant survival rate at 12 years was 94.6%  4.0% with failure for any reason as the end point and 98.3%  2.4% with mechanical failure as the end point.

 

Tabutin J, Banon F, Catonne Y, Grobost J, Tessier JL, Tillie B. Should we resurface the patella in total knee replacement? Experience with the Nex Gen prosthesis. Knee Surg Sports Traumatol Arthrosc. 2005;13:534-8.

 

This was an open, prospective multicenter study in which the authors examined the outcome with or without patella resurfacing in 5915 osteoarthritis cases using an implant with an anatomic trochlear design (Nex Gen). The incidence of patellofemoral pain at 2 years was more common in cases in which patellas were not resurfaced, even though this group had lower preoperative incidence of patellofemoral pain. The incidence of lateral patellar release was similar in both groups.

 

Victor J, Banks S, Bellemans J. Kinematics of posterior cruciate ligament-retaining and—substituting total knee arthroplasty: a prospective randomised outcome study. J Bone Joint Surg Br. 2005;87:646-55.

 

The authors compared the kinematic behavior of cruciate-retaining and cruciate-substituting total knee arthroplasty. At a follow-up of 5 years, kinematic analysis of lunge activity showed a mean posterior displacement of both medial and lateral tibiofemoral contact areas (roll-back) that was greater and more consistent in the cruciate-substituting than the cruciate-retaining group (medial, p  .0001; lateral,

p  .011). They also found that the roll-back was a predictable indicator of the maximum flexion that could be achieved (p  .018).

 

Whiteside LA, Arima J. The anteroposterior axis for femoral rotational alignment in valgus total knee arthroplasty. Clin Orthop Relat Res. 1995;(321):168-72.

 

The authors described a technique using the anteroposterior axis of the distal femur, rather than the transepicondylar or posterior femoral condylar axis, to establish rotational alignment of the femoral component in total knee arthroplasty. The anteroposterior axis of the distal femur was defined by a line through the deepest part of the patellar groove anteriorly and the center of the intercondylar notch posteriorly.