Minimally Invasive Surgery: Dr. Ries and Dr. Berend

Dr. Fehring: Minimally invasive total knee surgery was introduced about 5 years ago as a means to diminish pain and hasten rehabilitation. As problems with alignment, proper ligamentous balance, and wound healing have arisen since its inception, enthusiasm for this technique has waned.

Dr. Ries, is there still a place for minimally invasive techniques at this time?

Dr. Ries: The real risks and benefits of minimally invasive total knee arthroplasty (TKA) have been obscured by the effects of marketing minimally invasive surgery (MIS) TKA in clinical practice with broad claims about the benefits and little acknowledgment of risks. However, most studies of MIS compared with conventional TKA generally indicate that there can be some early benefit of faster recovery, a higher risk of complications, and no long-term benefit.^10,14,24,26

Prior to the introduction of minimally invasive surgery, I believe most surgeons, myself included, were trained to use a standard incision size and approach for all patients. I had used an incision from approximately 4 fingerbreadths above the patella to 2 cm distal to the tibial tubercle. Although this incision size may be necessary in obese patients, TKA can be safely performed in thin patients with good range of motion (ROM) and minimal deformity through a smaller incision.

The experience of MIS for me has been to emphasize the concept that the same incision length is not needed for all patients. I now begin my surgery with an incision from the superior pole of the patella to the tibial tubercle. If I am having any difficulty with the surgery, I then extend the incision. I consider minimally invasive surgery as the smallest incision that can be used to safely perform the TKA. When patients ask if I use MIS, I reply, “Yes, I use it routinely,” and I repeat my definition of it. In my experience, this approach has provided a very satisfactory end to the discussion for both the surgeon and patient.

Dr. Fehring: Is there an ideal candidate for minimally invasive total knee surgery?

Dr. Berend: I firmly believe that no total knee arthroplasty is as minimally invasive as the minimal operation would be to perform a compartmental arthroplasty with preservation of any normal structures. Therefore, in most knees, total knee arthroplasty is by definition not minimal. Assuming that we agree that minimally invasive total knee arthroplasty is likely a misnomer, the ideal candidate for less invasive arthroplasty would be a patient with minimal deformity, a supple flexible knee, and good range of motion. Patients with mild valgus deformity and mild lateral patellar subluxation appears to be the easiest patients to begin learning and gaining experience with the techniques for less invasive total knee surgery. Patients in whom concerns over less invasive techniques exist would be those with patella baja, previous surgery to the knee, significant intra-articular and extra-articular deformity, morbidly obese patients, and patients with previous issues of wound healing.

Dr. Fehring: What is your preferred approach to a normal-sized patient with minimal deformity?

Dr. Ries: I begin the surgery with the knee flexed 30 to 40 degrees, making an incision from the superior pole of the patella to the tibial tubercle, and raise a relatively wide medial skin and subcutaneous tissue flap and a smaller lateral flap. I believe it’s very important to dissect the subfascial plane carefully and maintain the full thickness of the subcutaneous layer to preserve the vascularity of the soft tissue flaps. I then make a medial parapatellar arthrotomy and dissect the medial tibial soft tissue sleeve subperiosteally to the midcoronal plane. Mid- or subvastus approaches have not been particularly useful for me in that these require more tension on the extensor mechanism to sublux the patella and can result in trauma to the vastus medialis. I then flex the knee fully, which retracts the proximal skin and subcutaneous soft tissues and exposes the rectus tendon. I extend the proximal arthrotomy into the rectus tendon, remove some portions of the fat pad that impinge into the knee, release the tibial insertion of the fat pad and lateral meniscus, and sublux the patella laterally without eversion.

I prefer to make the distal femoral cut first, followed by the tibial cut, and check the size of my extension space and alignment with a spacer block. Then I proceed with the remaining femoral cuts and TKA. The most helpful MIS technique for me has been the concept of a mobile window. I flex the knee fully while working on the tibia, place the knee at 90 degrees while working on the distal femur, and extend the knee to 60 to 80 degrees while working on the anterior femur.

Dr. Fehring: What is your preferred approach, Dr. Berend?

Dr. Berend: My preferred approach to a normal size patient with a minimal deformity would be to perform a less invasive, small incision, a modified mini-Tri-Vector. Through this approach, the vastus medialis is taken down from the extensor mechanism and the rectus tendon of the quadriceps is not involved in the dissection.

Dr. Berend, is there any evidence-based literature that documents improved function or quicker rehabilitation with MIS techniques?

Dr. Berend: There is clear evidence-based literature that supports the fact that partial knee arthroplasty does allow for quicker rehabilitation compared with total knee arthroplasty. Lombardi and colleagues,³⁰ in a retrospective study, has shown that patients undergoing partial knee arthroplasty recovered significantly faster than those undergoing a total knee arthroplasty. In terms of less invasive approaches to total knee arthroplasty as compared with standard approaches, the perioperative care of the patient is likely the most important factor. If we truly isolate the surgical technique away from the perioperative care protocols, recent literature from Tsuji and associates⁵² has shown that MIS TKA patients have an earlier, faster recovery than traditional total knee arthroplasty patients. In another recent, well-designed study, Dalury and coworkers¹⁵ have demonstrated that the invasiveness of the procedure as measured by the amount of tibial translation and patellar eversion does not affect the early postoperative recovery period after total knee arthroplasty. Therefore, it should be concluded that there are multiple factors at play when looking at the improved function and quicker rehabilitation with MIS techniques.

Dr. Ries, do you agree?

Dr. Ries: Most publications on this topic consist of observational case series, single-cohort studies, and expert opinions.¹⁰ However, several randomized prospective studies have been done that indicate either no benefit of MIS or faster recovery of early knee function in the MIS group.^24,26 Long-term benefits of MIS have not been demonstrated and there can be a greater risk of complications with MIS surgery.¹⁴

Custom Implants and Instrumentation: Dr. Berend and Dr. Lombardi

Dr. Fehring: A recent advance in knee arthroplasty has been the use of computed tomography (CT) or magnetic resonance imaging (MRI) data to make patient-specific implants and instruments. This technology is now offered by a number of orthopedic manufacturers.

Dr. Lombardi, what are the clinical advantages of this technique versus traditional total knee arthroplasty?

Dr. Lombardi: The use of CT or MRI data to make patient-specific implants and instruments has a number of clinical advantages. First and foremost, it takes preoperative planning to a totally different level. It requires the orthopedic surgeon to evaluate a three-dimensional model of each patient and develop a sophisticated preoperative plan. Restoration of the mechanical axis has been shown to be the key to success in total knee arthroplasty, especially with reference to enhanced durability.^7,13,32 The use of patient-specific guides has been shown to decrease the number of outliers and therefore enhance the quality of the reconstruction.²⁸ Obtaining the appropriate rotation of the femoral component at the time of surgical intervention has been shown to be keenly important to the success of the arthroplasty, especially with respect to the patellofemoral articulation.²⁵ With the three-dimensional reconstruction of the patient’s anatomy, the transepicondylar axis can be adequately identified in the preoperative plan. Computer navigation doesn’t offer the same degree of accuracy with respect to femoral component rotation. The use of patient-specific guides requires less instrumentation and, therefore, there is less potential for contamination of the surgical field. The use of patient-specific guides does not require violation of the femoral canal or the placement of pins in the femur and tibia. Finally, the operative intervention can be performed more efficiently with patient-specific guides because many decisions have been performed prior to the commencement of the surgical procedure.

Dr. Fehring: Do you share this enthusiasm?

Dr. Berend: Clinical advantages to any patient-specific technology are somewhat abstract at this time. It is, however, logical to assume that any instrumentation that allows more accurate implantation of the total knee arthroplasty device may reduce the incidence of either outliers or early failure. When comparing the MRI custom implant and instrumentation technology to that of traditional total knee arthroplasty, the bar has been set extremely high through previous implant and instrument designs and surgical techniques. However, the clinical advantages may be in more accurate implantation, possibly allowing for a lower incidence of early and midterm failure.

It has been our experience that custom MRI-based cutting guides for total knee arthroplasty have been advantageous in efficiency in the operating room and accuracy of femoral and tibial preparation.

Dr. Fehring: What are the economic implications of this technique versus traditional knee arthroplasty?

Dr. Berend: The economic implications are significant—the cost of the MRI, the establishment and creation of the custom instruments, and potentially the creation of custom implants. All add increased cost to an already expensive procedure. Extrapolating this to surgical navigation for total knee arthroplasty, we have shown that when compared with navigation, the use of this technology is much more cost-effective because of decreased operative time. However, compared with traditional knee arthroplasty, there are significant increases in cost. If this technology is able to reduce outliers and reduce the failure rate of total knee arthroplasty, the economic impact may be positive.

Dr. Fehring: Can we afford this in an era of declining health care resources?

Dr. Lombardi: Economic implications of the use of CT or MRI data to make patient-specific implants and guides involve the actual cost of this technology. Patients will need to obtain a preoperative CT or MRI scan. There is a cost associated with this. Furthermore, this information will be used to create a three-dimensional model and a patient-specific guide. The current economic burden for this technology is between $1000 and $1500 per case. The economic justification for this technique involves the following:

Dr. Fehring: Concerns about this technique have been voiced by surgeons who consider knee arthroplasty a soft tissue operation. This technique seems to focus solely on bony anatomy. What are your thoughts about this concern?

Dr. Lombardi: It is a well-known fact that the satisfactory performance of a total knee arthroplasty is a combination of bony resection and appropriate soft tissue reconstruction. The keys to success appear to be restoration of the mechanical axis and effective balance of the flexion-extension gap. Traditionally, surgeons have been offered a combination of intramedullary and extramedullary alignment guides to perform the appropriate bony resections. Computer navigation has also assisted the surgeon in performing the appropriate bony resections and has served to eliminate outliers.³⁷ Neither technique has specifically addressed soft tissue balance. The use of patient-specific guides brings the bony resections to a different level. It allows the surgeon to perform very precise resections and obtain appropriate alignment to reconstruct the mechanical axis and to position the femoral component appropriately with respect to rotation. These guides do not eliminate the need for the surgeon to be a surgeon and balance the soft tissues. However, it provides them with a set of instruments that are more accurate and less invasive than either the conventional instruments or computer navigation. With these guides, the surgeon can competently perform the bony resections and spend more time in balancing the soft tissues.

Dr. Fehring: Is this technique appropriate for severe deformities?

Dr. Berend: We have yet to determine whether this technique is appropriate for severe deformities. It would appear that it may be beneficial because our technique uses the mechanical axis. It may therefore be extremely helpful in these difficult cases. Time and experience will have to tell us whether this is true.

Bearing Surface Issues: Dr. Matsuda and Dr. Ries

Dr. Fehring: A number of different strategies have been used to diminish wear and osteolysis in total knee arthroplasty. Rotating platforms have been used for many years with the theoretical advantage of diminishing wear. Clinical studies documenting this improvement in wear are lacking; however, the theoretical advantage of linear wear at the articular surface and undersurface of the polyethylene remain encouraging. Countersurface wear through the use of ceramic-coated implants also has enticing theoretical advantages.

Dr. Ries: In your practice, do you use mobile bearings or alternative bearings to improve wear characteristics in your total knee patients?

The benefit of mobile bearings was most apparent when ultra–high-molecular-weight polyethylene (UHMWPE) was sterilized by gamma irradiation in air. Fixed-bearing, relatively nonconforming tibial components typically failed as a result of fatigue wear mechanisms (delamination and pitting). Nonconforming designs with low contact areas are associated with high contact stresses and, when coupled with the diminished ductility, wear resistance, and fatigue strength of UHMWPE, fatigue mechanisms occurred. Mobile bearings have highly conforming articular surfaces, which results in lower contact stresses. Fatigue failure of UHMWPE that was gamma-irradiated in air with mobile bearings has been extremely uncommon. Although wear can occur at both the upper and lower articulating surfaces of the tibial insert, the contact areas are high, resulting in contact stresses that are sufficiently low to reduce fatigue wear substantially. However, gamma-irradiated in air sterilization of UHMWPE was abandoned by manufacturers in the mid-1990s. Failure caused by fatigue wear (delamination and pitting) does not appear to be a clinical problem with currently available conventional UHMWPE tibial inserts (gamma-irradiated in an inert atmosphere, ethylene oxide, or gas plasma–sterilized). Therefore, it is not clear if there is much of a benefit to the use of mobile bearings with currently available conventional UHMWPE.

Wear is increased by counterface roughening, which can occur in vivo. This is why titanium, which is a relatively soft material, is not used as a bearing surface. Cobalt-chrome used in total knee femoral components is generally cast, which is not as hard as the forged cobalt-chrome used in total hip femoral heads. Cobalt-chrome femoral components in TKA can roughen during in vivo use. Surface hardening treatments such as diffusion hardening or ion bombardment can increase the hardness of the counterface. Ceramics are extremely scratch-resistant, but monolithic ceramics are brittle and can fracture in vivo. I use an oxidized zirconium femoral component, which has a ceramic surface and the strength of a metal, to reduce in vivo counterface roughening and wear. The only disadvantage I see with this material is that it costs more than cobalt-chrome.

Dr. Fehring: Is there any evidence-based literature that would support the use of these newer technologies?

Dr. Mastsuda: I have not seen any randomized control studies demonstrating better wear performance in the clinical use of mobile bearings or alternative bearings. I am currently doing a randomized control trial (RCT) using an identical design for both mobile and fixed bearings. This study has not shown any improvement with mobile bearings in wear or loosening at 5 years of follow-up.³⁸

Dr. Fehring: Why don’t we have an evidence-based answer for this question?

Dr. Ries: Evidence-based literature to support use of alternative bearings in TKA is lacking for several reasons. Evidence-based medicine generally requires that a statistically significant clinical outcome be demonstrated in randomized prospective studies between new and conventional technologies. However, differences in outcomes related to wear are not apparent for many years. By the time a clinical outcome such as failure caused by wear and osteolysis is demonstrated, the technology may be outdated and potentially newer and better technologies have been developed. In an effort to provide the best care for our patients, many of our clinical decisions are not based on evidence-based literature. There is a large body of literature demonstrating high survivorship with mobile bearings used with gamma-irradiated in air sterilized UHMWPE. I would expect that mobile bearings with currently available conventional UHMWPE will also have excellent long-term survivorship because conventional UHMWPE (gamma-irradiated in an inert environment, ethylene oxide, or gas plasma–sterilized) has more favorable wear behavior than UHMWPE that has been sterilized by gamma irradiation in air. However, it is not clear if currently available mobile bearings will provide an advantage over fixed bearings used with currently available conventional UHMWPE, because fatigue failure is unlikely to occur with either design.

There is a large body of in vitro wear testing data to support the use of a ceramic counterface in TKA, and the literature indicates that no adverse effects have occurred from the use of oxidized zirconium in TKA.^48–50 However, long-term clinical studies that demonstrate an advantage of a hardened counterface compared with a cast cobalt-chrome counterface are not available. Although wear can be measured radiographically in total hip arthroplasty (THA), there are no reliable radiographic methods to measure wear in TKA. Evidence-based studies that demonstrate an effect of new technology on wear in TKA will require an assessment of clinical failure caused by wear as an outcome measure. Because failure caused by wear occurs after many years of in vivo use, well-controlled, large, long-term clinical studies will be needed to determine the effects accurately of new technologies intended to reduce wear in TKA.

Dr. Fehring: Who is the most appropriate candidate for an alternative bearing surface or a mobile bearing?

Dr. Matsuda: I think that the most appropriate candidates for mobile bearings would be young, active patients requiring high flexion. Theoretically, we can expect low wear from these implants and young patients would benefit from this during their extended postoperative period.

Dr. Fehring: Do you concur?

Dr. Ries: The results of TKA with conventional materials can generally be expected to provide excellent survivorship of greater than 90% at 10 years. For older or inactive patients who would not be expected to outlive the longevity of their TKA, an alternative bearing is not necessary to reduce wear further.

However, because these technologies are also associated with increased costs, the technology is best suited for young active patients who may develop wear-related failure in their lifetime with the use of conventional materials.

Cross-Linked Polyethylene: Dr. Berend and Dr. Ries

Dr. Fehring: Cross-linked polyethylene has become the gold standard in hip arthroplasty. Clinical studies have verified the improved wear characteristics of cross-linked polyethylene in the hip. To date, such studies with regard to knee arthroplasty are lacking. Concern about the use of cross-linked polyethylene in the knee revolves around the mechanical toughness of cross-linked polyethylene in a different kinematic environment from hip arthroplasty.

What is different about the stress on the polyethylene in the knee versus the hip and why are mechanical properties of the plastic important in the knee?

Dr. Ries: The hip is a fully conforming joint with a large contact area. This results in low contact stresses, whereas the contact area in the knee is smaller and associated with higher contact stresses. The higher contact stresses in the knee have contributed to delamination and pitting observed with the use of UHMWPE that was gamma-irradiated in air and produce larger wear particles than in THA.

In THA, high stresses can also develop at the liner-locking mechanism from impingement of the neck on the acetabular rim. In high-flexion TKAs, cam post contact and femoral rollback in deep flexion results in increased stress on the post and insert baseplate locking mechanism. High strength and ductility of the UHMWPE are needed in TKA to minimize the risk of fatigue failure (delamination and pitting) and catastrophic failure of the posterior-stabilized (PS) post or insert baseplate locking mechanism.

Dr. Fehring: What concerns you most about using cross-linked polyethylene in the knee?

Dr. Berend: I have some concerns about post integrity. We know from some of the early experiences with cross-linked polyethylene in the hip that brittleness and crack propagation are significant issues, depending on the manufacturing process. In addition, in vivo oxidation may occur in some of the materials available. Therefore, the mechanical properties of the plastic may become extremely important in a PS or constrained-type knee design.

Dr. Fehring: Do you use cross-linked polyethylene in your practice today, and why or why not?

Dr. Berend: I’m just beginning to use cross-linked polyethylene in the knee as the newest generation of cross-linked polyethylene has become available. The new generation involves cross linking the polyethylene and then doping the material with tocopherol (vitamin E) to reduce the oxidation potential. Early biomechanical testing shows that the yield and tensile strength are unaffected by this manufacturing process, but the resistance to wear is significantly improved. Although this is an early experience, we have to be very vigilant with following these patients for any type of mechanical complication, early failure, or significant midterm issues with polyethylene degradation.

Dr. Fehring: How about you, Dr. Ries?

Dr. Ries: The clinical experience with the use of highly cross-linked UHMWPE in THA has been very good. Radiographic studies consistently demonstrate reduced femoral head penetration into the acetabular liner. However, occasional rim fractures have also been reported.^20,22,51 These appear to result from neck impingement and cantilever stresses on an unsupported elevated section of the rim above the metal shell. The post of a PS TKA also represents an area of unsupported UHMWPE when contacted by the cam, which could result in fracture. PS post fractures have occurred in vivo with both sterilized UHMWPE gamma-irradiated in air and gamma-irradiated in an inert atmosphere.^3,12 Because I routinely use a TKA having a cam-post mechanism, I have not used highly cross-linked UHMWPE in my TKA patients. However, there is a potential benefit of improved abrasion resistance and little risk of fracture using highly cross-linked UHMWPE in a posterior cruciate–retaining TKA because there is no cam-post mechanism.²³

Dr. Fehring: Are there patients for whom we should routinely use cross-linked polyethylene and, conversely, are there patients for whom we should avoid this type of polyethylene?

Dr. Ries: The experience from the use of highly cross-linked UHMWPE in THA and in vitro wear simulator studies in TKA indicate that wear in TKA can be reduced with the use of highly cross-linked UHMWPE. However, highly cross-linked UHMWPE has diminished mechanical properties compared with conventional UHMWPE. This risk needs to be assessed to determine if the risk-benefit ratio with the use of highly cross-linked UHMWPE in TKA is favorable.

In TKA, the stresses on the insert–baseplate locking mechanism would be expected to be higher with the use of a cam-post mechanism. For cruciate-retaining (CR) designs in which the anteroposterior (AP) tibiofemoral position is controlled more by ligaments and dynamic muscular forces, the insert locking mechanism stresses and risk of mechanical failure with the use of highly cross-linked UHMWPE would be expected to be low. For CR designs, the use of highly cross-linked UHMWPE appears to be safe.²³ For younger, more active patients who may experience failure because of wear and require future revision surgery, the use of highly cross-linked UHMWPE in CR TKA has a favorable risk-benefit ratio. For cam-post designs, it is difficult to assess the level of risk of post or insert locking mechanism failure. Modular implants with more articular constraint and cam-post mechanisms should be evaluated to determine if the insert-baseplate locking mechanisms and post designs are sufficient to be used with highly cross-linked UHMWPE. Highly cross-linked UHMWPE in these implants should probably be reserved more for young patients at high risk of failure because of wear and avoided in patients at high risk of failure because of mechanical overload, such as overweight active patients, until more information is available about the safety of these designs when used with highly cross-linked UHMWPE.