44Minimally Invasive: Total Knee Arthroplasty 44

Summary

Numerous long-term clinical studies have demonstrated the success of total knee arthroplasty in regard to im- proving both pain and function. The typical exposure for a total knee arthroplasty employs an extensile approach through a medial parapatellar arthrotomy. The incision through the quadriceps tendon can lead to a long and painful recovery. The authors began performing mini- mally invasive total knee arthroplasties through a modi- fied midvastus approach in 2001 as an alternative to the traditional approach. If the patella is not everted or the quadriceps tendon not transected, the extensor mecha- nism suffers less surgical trauma. Total knee arthroplas- ties done utilizing this technique achieve not only knee motion faster, but also a greater ultimate range of motion without an increase in complications.

Introduction/Background

Total knee arthroplasty (TKA) is a very successful treat- ment for end-stage arthritis of the knee. Long-term re- sults for both pain relief and function are excellent [1-4].

The approach most commonly used to expose the knee is the medial parapatellar arthrotomy [5]. This approach was first described by Von Langenbeck in 1874 [6], and it has a successful track record with long-term follow-up.

However,the incision through the quadriceps tendon and patellar eversion can lead to a long and painful recovery.

Another approach used for TKA, the midvastus ap- proach, does not require incision of the quadriceps ten- don [7]. Several studies have demonstrated a number of clinical improvements with this approach when directly compared with a standard medial parapatellar arthroto- my, including faster recovery of quadriceps strength, less postoperative pain, and a lower lateral retinacular release rate [8-12]. While the standard midvastus approach had numerous benefits, it still involved patellar eversion, which in some cases was difficult to perform. Eversion of the patella also leads to more splitting of the muscles and stretching of the quadriceps and patella tendons.Eversion of the patella via a medial parapatellar or midvastus ap-

proach also causes twisting of the patella tendon and non- physiological stress at the tibial tubercle. In some cases this leads to pealing or avulsion of the tendon.

In 2001, in order to decrease the trauma to the exten- sor mechanism, the authors began performing minimal- ly invasive total knee arthroplasty (MIS-TKA) utilizing a modified midvastus approach. We named it the “mini- midvastus” approach. The modification involved in this approach is that the patella is not everted, but rather dis- located laterally during the procedure. By not everting the patella, it is not necessary to split the VMO as far me- dially, and less stress is placed on the extensor mecha- nism.

Another potential advantage of this exposure is that it can be performed through a smaller skin incision. The incision with a traditional medial parapatellar arthroto- my must be carried proximally to the extent of the split in the quadriceps tendon. This is not necessary with the mini-midvastus technique. In fact, we have found that, with improvements in instrumentation, a total knee arthroplasty can be safely performed through an 8.5- to 12-cm skin incision without excessive retraction or the need for undermining the skin.

Instrumentation

Traditionally,instruments used for TKA have dictated the length of the incision. Smaller, modified instrumentation has been an integral part in the development of the MIS- TKA technique. Not only were the guides and cutting blocks made smaller; the corners of instruments were rounded and separate sets were made for both the right and the left knee.

The first change made was to the tibial cutting guide.

Because of the proximity of the patellar tendon, the later- al wing of the tibial cutting guide is rarely used during the tibial cut with standard instrumentation. Hence, the tib- ial guide was made without a lateral wing. Instead, both a left and a right guide were made to wrap around the me- dial aspect of the tibia (

). This modification al- lows the lateral tibial plateau to be cut from anterior to posterior as well as from medial to lateral.

44 Minimally Invasive:

Total Knee Arthroplasty

S. B. Haas, A. P. Lehman, S. Cook

The femoral cutting blocks and valgus alignment guides were also modified.Again, a separate guide was made for each side. The anterior femoral cutting guide, distal femoral cutting block, and the 4-in-1 femoral finishing block were made narrower in the medial-to-lateral di- mension and the corners were rounded (

). This allows easy placement of the guides without impingement on the patella laterally.

The remainder of the instruments, including the anteri- or resection stylus, the distal resection stylus, the hous- ing resection block for posterior stabilized knees,and the femoral sizing guide, are also smaller in order to fit a smaller incision. Additionally, the anterior stylus is an- gled to allow placement under the skin proximally when referencing the anterior femoral cut, and the distal femoral cutting block is wedge-shaped and retracts the skin edges without the need for additional retractors (

).

Surgical Technique

With the leg fully extended, a longitudinal incision is made over the anterior aspect of the knee along the me- dial border of the patella. The incision is generally be- tween 8.5 and 12 cm and extends from the superior pole of the patella proximally to the proximal half of the tibial tubercle distally.If additional exposure is required,the in- cision can begin 1 cm proximal to the superior pole of the patella. A medial arthrotomy is then performed from the proximal border of the patella to about 5 mm medial to the tibial tubercle. The suprapatellar pouch is identified, separated from the underside of the quadriceps tendon, and preserved. The distal extent of the VMO is identified at the superomedial corner of the patella. The fascia of this muscle is incised obliquely along the line of the mus- cle fibers for approximately 1-2 cm (

). The mus- cle fibers are then bluntly spread by hand.

While the leg is kept fully extended, the patella is re- tracted laterally. A portion of the fat pad is excised both medially and laterally. The anterior horn of the medial meniscus is divided and excised.Subperiosteal dissection is performed around the proximal medial tibia in stan- dard fashion. The anterior cruciate ligament and the an- terior horn of the lateral meniscus are excised. A small

bottom

⊡ Fig. 44-1.(top) Standard Genesis II (Smith and Nephew, Memphis, TN) tibial cutting block. (bottom) MIS Genesis II (Smith and Nephew, Memphis,

TN) tibial cutting block ⊡ Fig. 44-3. Preparing femur with MIS distal cutting block

top

bottom

⊡ Fig. 44-2. (top) Standard Genesis II (Smith and Nephew, Memphis, TN) femoral A-P cutting block. (bottom) MIS Genesis II (Smith and Nephew, Memphis, TN) femoral A-P cutting block

window is made along the anterior surface of the distal femur in order to reference the anterior cortex.

The TKA continues in routine fashion from this point.

In patients with tight extensor mechanisms, an abun- dance of patellar osteophytes, or large patellae, the patel- la can be cut first to facilitate exposure. However, in pa- tients with osteoporotic bone,care must be taken to avoid intraoperative damage to the cut patella surface with re- tractors. If it is not opted to cut the patella first, the femur should be prepared first with the patella retracted later- ally but not everted.

Importantly, femoral preparation should be accom- plished with the knee flexed to only 70°-90°. Limiting knee flexion allows the soft-tissue window to be mobile, and thus to move proximally without difficulty in order that the anterior femoral cortex can be referenced. Hy- perflexion should be avoided, because this will not only tighten the extensor mechanism but also limit exposure.

A thin, bent Hohman retractor is placed laterally to re- tract the patella.A 9.5-mm intramedullary drill is used to enter the femoral canal from a starting point in the notch just anterior to the posterior cruciate insertion on the fe- mur.An intramedullary referencing guide is placed in the femoral canal after the marrow contents have been sucked out. The appropriate valgus angle collar, with or

without posterior condylar referencing paddles, should be placed in the rod. If the posterior paddles are utilized, the knee must be flexed to 100° during insertion.After ro- tation is set,the posterior paddles are removed and the leg is returned to between 70° and 90° of flexion.

Rotation for the anterior cut is determined utilizing the traditional landmarks of the anteroposterior axis (Whiteside’s line), the epicondylar axis, or the posterior condylar axis (

). The authors prefer to use Whiteside’s line as the primary reference to femoral rota- tion and the posterior condylar axis as a secondary refer- ence.

Following determination of femoral rotation, an an- terior stylus is used to reference the preliminary anterior femoral resection. The stylus is placed in the small win- dow that was created during exposure and flush on the highest point of the anterior femoral cortex. The saw blade will cut under the skin, but a right-angle retractor may be used to elevate the skin if desired.

At this time,the distal femoral resection is performed.

Additional retractors are usually not necessary for place- ment of the guide. Because of the guide’s wedge shape, it typically will provide adequate retraction by itself. Once the cutting block is placed on the anterior femur and se- cured in place, the distal femoral cut is made.

The size of the femoral component is determined us- ing a sizing guide. The knee may require more flexion in order to place the guide in the correct position under the posterior condyles. The appropriately sized femoral cut- ting block is placed on the distal femur and secured. A thin, bent Hohman retractor is placed medially to protect the medial collateral ligament, as well as laterally to re- tract the patella. The femoral cuts are made in the follow- ing order: posterior condyles, posterior chamfer, anteri- or resection, and anterior chamfer.

44

⊡ Fig. 44-4. Arthrotomy with mini-midvastus approach extends from tibial tubercle to superior patella and then to the muscle of the vastus me- dialus. The muscle fibers are not cut

⊡ Fig. 44-5. Preparing femur with right-sided MIS valgus/rotation guide.

The stylus is aligned with the AP axis line (Whiteside’s line)

The tibia is prepared next. Either intramedullary or extramedullary tibial alignment guides can be utilized.

The authors prefer an extramedullary technique.We typ- ically reference the guide over the tibial crest proximally and over the second metatarsal distally.A stylus is used to measure the amount of tibia to be removed from the least deficient tibial condyle. For a varus knee, the stylus is placed on the anterior aspect of the lateral tibial plateau.

In general, we set the stylus to measure 11 mm of proxi- mal tibial bone resection. Again, the two thin, bent Hohman retractors are in place medially and laterally, protecting the medial collateral ligament and the exten- sor mechanism, respectively.An Aufranc retractor is also placed posteriorly. The saw blade should initially be di- rected in a posterior direction. Then it should be direct- ed laterally.

After the tibial cut is made, extension of the knee can provide a better perspective when the cut surface is in- spected for cortical ridges.After the tibial bone is removed, the posterior horns of both the medial and lateral menis- ci are removed along with the posterior cruciate ligament (if a posterior-stabilized knee system is being utilized).We prefer to place a laminar spreader in the joint while the knee is in 90° of flexion in order to distract the surfaces and provide exposure of the posterior elements of the knee. Any posterior osteophytes should be removed with an osteotome at this time (

). The tibial prepara- tion should be completed by measuring the surface and drilling and broaching the tibial canal to fit the prosthesis.

Spacer blocks are placed at this time in order to mea- sure the extension and flexion gaps. If there is asymme- try, the appropriate releases should be performed to cre- ate a balanced knee. If a posterior-stabilized knee is be- ing used,the posterior-stabilized resection block is placed on the distal femur and the femoral preparation is com- pleted at this time.

Following tibial preparation, the patella is prepared.

Unless it is required earlier for adequate exposure, it is

easier to wait until after both the femoral and tibial cuts have been made to perform this step.With the knee in full extension, the leg is shortened and the extensor mecha- nism is relaxed. The patella is easily rotated 90°s and cut in routine fashion (

).

Once all the surfaces have been prepared, the trial components are placed. In order to clear the femoral condyles with the tibial trial, the knee must be maximally flexed, and in addition to thin, bent Hohman retractors medially and laterally, an Aufranc retractor can be placed posteriorly to subluxate the tibia anteriorly. The femoral trial is placed with the knee in 90° of flexion.Once the size of the polyethylene insert is confirmed and the placement of the trial is deemed satisfactory, the trials may be re- moved and the final implants placed using the same tech- nique.The authors prefer to use cemented implants; how- ever, cementless implants can also be used. The tibial component should be placed first in order to have access to the posterior aspect of the knee to allow complete re- moval of excess cement. The use of an asymmetric, anatomical tibial component is recommended, since the larger lateral side of the tibial component in symmetric designs often leads to impingement on the lateral femoral condyle during insertion, posterolateral overhang, and difficulty with cement removal. Once the tibial compo- nent has been placed, the femoral and patellar compo- nents are placed.Retraction of the proximal tissues for ce- ment removal must be avoided with the knee flexed. Ex- cess cement anteriorly in the suprapatellar pouch can be more easily removed with the leg in extension. The trial polyethylene insert is placed, and the leg is kept in full ex- tension to pressurize the cement while it is polymerizing.

The final polyethylene insert is placed after the cement has hardened. If a posterior-stabilized insert is used, in-

⊡ Fig. 44-6. Laminar spreader is placed after bone resection. Good vi- sualization for resection of meniscus and PCL and for posterior osteo- phyte removal

⊡ Fig. 44-7. Patella rotated approximately 90° for patella preparation.

Knee is extended, and femoral and tibial bone cuts have been made

into full extension to engage the locking mechanism. We prefer to use a high flexion insert in most cases.

The authors recommend that the tourniquet be de- flated after the cement has polymerized in order to achieve hemostasis. Once all bleeding has been con- trolled,the wound is copiously irrigated and closure is be- gun.Two deep drains are placed.The arthrotomy is closed by initially placing 0-Vicryl sutures in the VMO fascia and tendon. Three to five sutures will usually suffice. The remainder of the arthrotomy, the subcutaneous tissues, and skin are closed in standard fashion.

Results

The initial 40 MIS-TKAs performed by the senior author (S.B.H.) were evaluated and compared with an age- and sex-matched cohort (control group) of patients who re- ceived a TKA through a standard medial parapatellar arthrotomy by the same surgeon [13].All patients received the same posterior stabilized condylar knee (Genesis II, Smith and Nephew, Memphis, TN). Both groups also re- ceived the same postoperative physical therapy protocol, including a continuous passive motion machine in the re- covery room.

There were no statistically significant demographic differences between the two groups. The Knee Society preoperative knee and function score averages were 28 and 23, respectively, in the MIS-TKA group and 33 and 24 in the control group. At 1 year postoperatively, the aver- ages were 97 and 92 in the MIS-TKA group and 91 and 90 in the control group.

Preoperative range of motion was similar in both groups. However, at 6 weeks postoperatively, the mean flexion was significantly higher in the MIS-TKA group (114°) than in the control (96°). This difference persisted at 1 year, when the mean flexion in the MIS-TKA group improved to 125° and the control group improved to only 116°.

Although most patients selected for MIS-TKA during the first year weighed fewer than 100 kg, 10% were heav- ier. Over the past 3.5 years we have refined the selection criteria.Patients weighing less than 90-100 kg are the best candidates for MIS-TKA and women are generally easier to treat than men. Large muscular men are the most dif- ficult and are generally more difficult than overweight women.A standard medial parapatellar approach is used for patients with preoperative flexion contracture greater than 20° and flexion less than 80°. Poor-quality skin or multiple previous incisions generally require a larger in- cision; however,the MIS arthrotomy may be utilized.Dur- ing the past year, over 70% of all TKAs were performed using an MIS/mini-midvastus approach.While larger and

many of these patients to have TKA performed through less extensive medial parapatelar arthrotomy without patellar eversion and with a smaller incision.

Between September of 2001 and September of 2003, the senior author performed 389 MIS-TKAs using the mini-midvastus approach. This represented 66% of all TKAs performed during that time period. The mean weight and height were 79.5 kg (46-126) and 165 cm (132- 187 cm) and preoperative flexion was 109° (80°-150°). The mean postoperative flexion at 6 weeks and 3 months was 111° and 121°,respectively.One- and 2-year results with this larger group of patients were consistent with those of our earlier study. Range of motion at 1- and 2-year follow-up was 125°. The early functional results have been similar to those of the initial 40 patients. However, the operative time has been reduced significantly. The mean tourni- quet time is 48 min,which is statistically equivalent to our standard approach. Overall operating time is slightly less than the standard approach due to reduced time for clo- sure. The overall complication rate remains low. There has been a 0.5% incidence of infection and a 0.2% inci- dence of superficial wound necrosis.Lateral releases were performed in only 2% of cases. There have been no asep- tic failures to date (S.B. Haas, unpublished data).

Laskin et al.[14] performed a similar study using cru- ciate-retaining knees (Genesis II, Smith and Nephew, Memphis, TN). Thirty-two MIS-TKAs were compared with 26 standard TKAs. Preoperative patient demo- graphics, Knee Society Scores, knee flexion, and knee alignment were similar between the two groups.

The patients who underwent an MIS-TKA had a sta- tistically significantly lower amount of pain on a visual analogue scale in the postoperative period. Additionally, the average total dose of morphine sulfate was signifi- cantly lower for patients who had had an MIS-TKA (55 mg) than for those who had undergone a standard medial parapatellar arthrotomy (118 mg).

Passive knee flexion was also consistently higher in the MIS-TKA group. On the third postoperative day, 80%

of the patients who had had an MIS-TKA were able to achieve knee flexion of greater than 80°, while only 4 % of those who had had a medial parapatellar arthrotomy were able to achieve this amount of knee flexion.Also,the average knee flexion at 6 weeks postoperatively was sig- nificantly higher in the MIS-TKA group (124°) than in the control group (115°).

There was no statistical difference in either the radi- ographic position of the components or the postoperative leg alignment. No lateral retinacular releases were per- formed in either group. The mean tourniquet time in the MIS-TKA group was 58 min, compared with 51 min in the control group.There were no reported skin complications in either group.

44

Conclusion

The use of the mini-midvastus approach for MIS-TKA allows patients to regain knee flexion faster, and it may lead to a greater ultimate range of motion. It also leads to higher Knee Society Scores and decreases the patients’

postoperative pain.Smaller instruments allow the surgeon to decrease the amount of surgical dissection and the size of the incision without excessive soft-tissue retraction or additional complications. By not everting the patella or disrupting the suprapatellar pouch in the MIS-TKA, im- provement in patient recovery,as well as an improved cos- metic result, can be achieved without compromising the radiographic positioning of the implants, the clinical re- sults, or surgical exposure.

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