**8. Authors' preferred surgical technique**

In conclusion, evaluating the tibial slope is a crucial phase in CR-TKR, since as recently explained by Dae Kyung Bae et al. the steepness of the PTS is one of the main factors that

For a successful TKR, the wide range of movement is not the only objective to achieve, but also joint stability plays a crucial role. During most activities of daily living, the knee is loaded not only in full extension but also in midflexion, and therefore, stability throughout flexion should be considered an important outcome measure. As the PCL is thought to serve as a secondary stabilizer of the knee during varus and valgus stresses, it is likely that the presence

In a cadaveric study of 1999, Mihalko observed "…that a major result of posterior cruciate ligament sacrifice is the creation of a larger flexion gap. This result provides insight into relative joint line changes that can occur after posterior cruciate ligament sacrifice…," suggesting… "the need for greater attention to flexion stability when sacrificing the posterior cruciate ligament" [18]. In a 2008 study, Tsuneizumi et al. similarly concluded that "…the PCL kept the knee stable against distal traction force in the flexion position, and sacrifice of this ligament caused joint laxity in different ranges. The increases in the flexion gap after resection of

In 2013, Hino et al. focused on stability through the range of movement pinpointing the exact degrees that make differences between CR and PS TKRs stability: "Specifically, CR knees had significantly less laxity in the flexion range of 10 to 30° than PS knees…" demonstrating "…a significant decrease in joint laxity at 120° of flexion for CR-TKRs and in contrast PS TKRs had an increase in joint laxity between 10 and 20° of flexion post-operatively. Overall, CR knees demonstrated less joint laxity than PS knees throughout the whole range of movement." They concluded that "CR knees have less post-operative laxity, especially in deep flexion…" and "...this may be associated with the lower flexion range that could be seen in CR knees" [20].

Since restoration of joint line can be difficult in severe osteoarthritic knee with coronal and sagittal plane deformities, many surgeons prefer the use of a PS TKR, which is less sensitive to changes in joint line position: in fact, the level of the reconstructed joint line is one of the main factors that affects the tension of the retained PCL [21]. As reported by Emodi et al. in a cadaveric study "As the joint line was elevated, PCL strain increased at all measured flexion angles above 30°…" and "…the centre of tibio-femoral contact did not change at the flexion angles of 15 and 30°…" but "…at 60, 90 and 105° the tibio-femoral contact centre moved posteriorly with each successive elevation of the joint line." This author also confirmed what already reported in literature that "…Significant decreases in flexion were observed with as little as 2 mm of elevation. Flexion was limited further as the joint line

requires the conversion to PS from a CR-type prosthesis [17].

of this ligament has some effects on midflexion stability.

the PCL varied among individuals" [19].

**7. Joint line position**

**6. Midflexion stability**

28 Primary Total Knee Arthroplasty

A standard midline longitudinal approach is performed with medial parapatellar arthrotomy and lateral patellar dislocation; the knee joint is exposed and the Whiteside line and the transepicondylar axis are marked (they are used as femoral rotational landmarks); the intramedullary femoral guide is drilled into the femur; a 9-mm distal femoral bone resection is performed with a valgus angle preoperatively planned (**Figure 3**).

For the tibial side, Hohmann retractors are employed to protect lateral and medial soft tissues (critical structures for a correct balancing); the tibial spine is removed using a surgical saw.

Then through the use of an osteotome, a bone island really closer to the distal insertion of PCL is circumscribed (**Figures 4** and **5**). A pin is placed in front of the PCL defining the anterior side of the island (**Figure 6**); we make use of the extramedullary tibial resection guide to align the tibial mechanical axis; an amount of resection equal to the thickness of the tibial arthroplasty component is measured on the less osteoarthritic plateau and tibial resection is performed bewaring not to get across the lateral and medial sides of marked bone island. The employment of the osteotome is an essential step: it allows to incise the posterior cortex avoiding during the tibial plateau excision the risk of avulsion and subsequent detachment of the PCL.

With these precautions, the tibial plateau is removed (**Figures 7**–**9**) and the bone island can be carefully shaped through a nibbler to permit the placement of the posterior side of the tibial component (**Figures 10** and **11**).

**Figure 3.** Distal femur cut: with the angel wing, you can appreciate the minimal resection in order to maintain the level of the joint line position.

**Figure 6.** A pin is placed in front of the PCL defining the anterior side of the island.

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**Figure 7.** The tibial plateau is detached.

**Figure 8.** The tibial plateau is removed.

**Figure 4.** Through the use of an osteotome, a bone island is circumscribed.

**Figure 5.** The incision of the posterior cortex necessary to avoid a posterior avulsion.

**Figure 6.** A pin is placed in front of the PCL defining the anterior side of the island.

**Figure 7.** The tibial plateau is detached.

**Figure 4.** Through the use of an osteotome, a bone island is circumscribed.

30 Primary Total Knee Arthroplasty

**Figure 5.** The incision of the posterior cortex necessary to avoid a posterior avulsion.

**Figure 8.** The tibial plateau is removed.

**Figure 9.** Superior view of the removed plateau.

Through the use of a gap spacer, the alignment and varus or valgus ligamentous balance are always tested both in extension and in flexion. The space between the femoral and the tibial cut surfaces should be within 1–2 mm of each other both in flexion and

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**Figure 11.** The bone island can be shaped through a nibbler or a Luer.

**Figure 12.** Balancing the space in flexion through the flexion spacer by Biomet.

extension [23].

**Figure 10.** The aspect of the tibial side after the cut with the preservation of the PCL.

The following step, with knee flexed at 90°, is defining the size and the position of the femoral component; we utilize an asymmetric spacer (flexion spacer by Biomet) (**Figure 12**) to perform a posterior condylar femoral resection perpendicular to the Whiteside line and in order to obtain equal medial and lateral gaps and to achieve the same space both in flexion and extension. By posterior referencing, 4-in-1 cutting block (set for the measured implant size and the rotational alignment previously evaluated) is placed and the anterior, posterior, anterior chamfer, and posterior chamfer cuts are performed.

**Figure 11.** The bone island can be shaped through a nibbler or a Luer.

Through the use of a gap spacer, the alignment and varus or valgus ligamentous balance are always tested both in extension and in flexion. The space between the femoral and the tibial cut surfaces should be within 1–2 mm of each other both in flexion and extension [23].

**Figure 12.** Balancing the space in flexion through the flexion spacer by Biomet.

The following step, with knee flexed at 90°, is defining the size and the position of the femoral component; we utilize an asymmetric spacer (flexion spacer by Biomet) (**Figure 12**) to perform a posterior condylar femoral resection perpendicular to the Whiteside line and in order to obtain equal medial and lateral gaps and to achieve the same space both in flexion and extension. By posterior referencing, 4-in-1 cutting block (set for the measured implant size and the rotational alignment previously evaluated) is placed and the anterior, posterior, anterior

chamfer, and posterior chamfer cuts are performed.

**Figure 10.** The aspect of the tibial side after the cut with the preservation of the PCL.

**Figure 9.** Superior view of the removed plateau.

32 Primary Total Knee Arthroplasty

When the desired balancing and gaps are achieved, the femoral, tibial, and insert trial components of the correct size are inserted; we check the maximum extension and flexion and then stability and the patellar tracking in the full range of movement; usually, we do not perform patellar resurfacing [24] but just remove osteophytes.

The third element is about the slope of the tibial cut: posterior slope opens the flexion space and this helps to obtain flexion without PCL recession. Therefore, a modest posterior slope (typically matching the patient's native slope within the range of 3–7°) may help to reduce tension on the PCL and this may facilitate knee flexion. If the extension gap is asymmetrical, it is necessary to perform a ligamentous release in order to obtain a symmetric space, and the gap size finally should be verified with a spacer block. Reflecting on the solution to achieve a symmetrical space in the flexion gap, it is really important to consider what type of instrument we are using: if we get the restoration of a neutral mechanical alignment in extension through the use of an intramedullary device for the femur and an external guide for the tibia, in our experience, the best instrument that helps us to get the correct rotation of femur in flexion is the flexion spacer by Biomet; in fact, it is always referred to the tibial osteotomy (that regardless of surgeon accuracy should be 1° or 2° in valgus or varus deviation) and allows us to reach a parallel cut even in case of a severe lateral condyle hypoplasia thanks to its multiple choice in asymmetrical components (1°, 3°, 4°, 5°, 6°, 8°). Once rotation is obtained, the following step is to determine the depth of the flexion gap. Not only does the flexion gap need to be rectangular (indication of rotation), but it also needs to obtain the same size of the extension gap. Through the use of the flexion spacer, the normal posterior condyle offset is recreated. Then, the depth of the anterior cut is not measured directly but rather determined by the size of the 4-in-1 cutting jig; it may be too shallow (this will cause overstuffing of the patellofemo-

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Nevertheless, problems can occur once that bone cuts are performed and trial components

A slightly lax PCL is preferable to one that is excessively tight, but an overly lax PCL can result in functional disability secondary to flexion instability. Balance of the PCL should be assessed after correction of any varus or valgus ligamentous imbalance. An excessively tight PCL will result in anterior translation of the tibia from beneath the femur, anterior lift-off of the trial polyethylene from the tibia tray in flexion, and/or displacement of the femoral component in flexion. A useful test of the relative balance of the PCL is the so-called POLO (for PullOut, LiftOff) test introduced by Dr. Richard Scott. In this test, a trial reduction is done with a stemless tibial trial and a curved tibial insert. The PullOut portion of the test is done at 90° of flexion and confirms that the PCL is not too loose if the tibial insert cannot be subluxed (pulled out) anteriorly from beneath the femur. The LiftOff portion is done while putting the knee through a range of motion up to 120° and ensuring that the tibial insert does not look open (lift-off) in flexion, indicating that the PCL is too tight. Scott postulates that if the PCL is

If the PCL is excessively tight, the tension can be decreased by several techniques. Increased tibial bone resection is only appropriate if the knee is tight in both flexion and extension. If the knee is tight only in flexion, increasing tibial bone resection will leave the knee lax in extension, resulting in symptomatic instability due to hyperextension or excessive varus-valgus play. If the knee is tight only in flexion, the posterior slope of the tibial cut should be assessed. The tibia normally has a 3 to 7°degree posterior slope. The amount of posterior slope cut on the tibia will be dependent on the prosthetic design. Some implants have an inherent posterior slope in the articular geometry and will require less posterior slope than knees with a flat

ral joint) or it may be too deep (this will cause notching) [26].

not too loose and not too tight, then it must be just right [1].

are inserted.

Once the tibial stem hole is created and the preparation procedure is completed, we perform a wash of the joint and then insert definitive cemented components and the polyethylene bearing of the previously evaluated size.
