Minimally invasive knee replacement surgery has been developed in an attempt to lessen the impact of operations on the patient’s quality of life, in the same way as arthroscopy, the fore-father of MIS, revolutionised cartilage and ligament surgery three decades earlier. The technique is based on minimising soft tissue and muscle trauma, skin incision length, and capsular disruption while trying to maintain the ultimate goal of a well aligned, well fixed knee replacement. Short-term advantages including accelerated recovery and reduction in blood loss have been demonstrated, but it remains to be investigated whether MIS is able to provide sustainable benefits and long-term outcome equivalent to conventional surgery.
Successful application of MIS techniques has been confirmed in conjunction with UKR implantation, whilst its use with TKRs remains experimental. Critics have expressed concern that reduced visibility may compromise implant positioning and leg alignment which could have a negative effect on prosthetic long term survival. MIS clearly represents a unique and more technically demanding procedure, but the learning curve may perhaps overshadow the benefits. Computer navigation may help to avoid such problems but so far scientific evidence regarding definitive outcomes is lacking, and some of the MIS techniques are still in the early phases of development.
1. INTRODUCTION
Key-hole surgery of the knee or otherwise known as arthroscopy revolutionised knee surgery in the late 1970’s and has to be considered the first successful attempt of applying minimally invasive principles to knee surgery. Although at first used as a diagnostic procedure arthroscopic applications have been widened to include complex ligament reconstructions and patellar re-alignment procedures.
The term 'minimally invasive surgery' or MIS was coined in the late 1990’s in order to describe a trend to reduce the length of the surgical incision and associated soft-tissue trauma during knee replacement surgery (Repicci 1999). MIS is relatively new to the orthopaedic community but has already gained in popularity through its successful application in conjunction with unicompartmental knee arthroplasty. The evolution of MIS to total knee arthroplasty however, appears to require an entirely new surgical technique rather than the evolution of a conventional one with well-established track-record.
MIS has generated great interest among patients and surgeons alike and has been subject to widespread media coverage. The lay press in particular has for some time been shaping patient’s expectations and demands based on the promise of a quicker, less painful recovery, earlier mobilisation, and reduced hospital stay by obtaining the same or better long-term results. Those expectations however may not always be realistic or fail to take into account the somewhat experimental nature of MIS.
2. ARTHROSCOPY
The first wave of minimally invasive surgery in orthopaedics dates back to the beginning of 20th century, when visualisation of the knee cavity was first performed by endoscopic means. Eugen Bircher a Swiss army surgeon published his early experience in 1921, creating the term ‘arthroscopy’. He was a visionary man who wrote at the time ‘Arthroscopy is superior to all other methods of investigation and like endoscopy of the bladder, can be used to define certain indications for surgery. It will be met with resistance but will gain in popularity and develop to the point at which it becomes indispensable’ (Bircher 1921). It took however more than half a century before arthroscopic surgery became widely accepted and readily available (Jackson & Abe 1972). The invention of the rod lens system and the glass fibre optic by Hopkins in the 1950’s greatly enhanced the quality and visibility by providing a significant reduction in instrument size (Ellis 2007). The final breakthrough came with the introduction of the video camera system at the end of the 1970’s, which was linked with an external monitor (Löhnert & Raunest 1988). This gave surgeons the opportunity to visualise the joint without the risk of de-sterilisation and paved the way to the development of operative arthroscopy. Today we perform cruciate ligament reconstructions, repair menisci and cartilage, remove loose bodies and debride arthritic knees routinely through the arthroscope, allowing patients to benefit from a fast postoperative recovery and early return to sporting activities (Jackson 2003).
3. UNICOMPARTMENTAL KNEE ARTHROPLASTY
A unicompartmental knee replacement is a surface replacement of opposing articulating areas of femur and tibia (Fig. ) (Scott 2002). The ideal patient for a UKR presents with localised wear in a single tibio-femoral compartment whilst the contra-lateral compartment and the cruciate ligaments are well preserved. Functionally, UKRs are considered superior to total knee arthroplasty as they provide mere physiological knee kinematics. This is mainly achieved through maintaining the patient’s pre-operative range of motion and retaining some of the proprioceptive qualities, which make the knee ‘feel more natural’ (Goodfellow & O'Connor 1986). The implantation of a unicompartmental knee replacement is technically quite demanding, as component alignment is crucial for long-term success (Sculco 1994). Early failures are mostly related to over-correction, implant mal-positioning and the use of polyethylene tibial components of 6 mm or less in thickness (Marmor 1988). Clinical results show that unicompartmental knee replacements are best suited for localised medial compartment osteo-arthritis, whilst its application for the lateral compartment arthritis provides a less predictable outcome (Ashraf 2002). So far long-term survival rates of up to 98% at 10 years have been reported with conventionally implanted medial compartment UKRs (Ansari et al 1997, Murray et al 1998, Robertsson et al 2001).
MIS technique for unicompartmental knee arthroplasty (UKR) was first performed in 1992 by John Repicci from Buffalo, New York (Repicci & Eberle 1999). Although the implantation of the components still required surgical opening of the knee joint, otherwise known as arthrotomy, the incision length has been reduced from 10” to as little as 3” in slim individuals (Fig. ). MIS technique allows visualisation of the treated compartment only, making it impossible to assess the remaining aspects of the joints during surgery. Henceforth the decisions upon patient’s suitability for partial rather than total knee arthroplasty has to be ascertained through pre-operative investigations such as MRI, stress radiographs and arthroscopy.
Within the UK there is growing acceptance in using MIS for the implantation of UKRs. Most manufacturers are now providing specialised MIS equipment helping to achieve reliable and reproducible results. Surgeons may elect to perform the operation through a limited (4” to 5”) or mini exposure (2” to 3”) depending on personal preference, experience and patient specific circumstances (scars from previous surgeries, patella baha etc.). Short to medium term follow-up studies have so far presented excellent results (Price et al 2001, Romanowski & Repicci 2002, Carlsson et al 2006). It has generally been recommended, that surgeons inexperienced with UKR surgery should at first use a wide incision before embarking on MIS techniques, as complications might otherwise occur (Fig. ).
4. TOTAL KNEE ARTHROPLASTY
The first reliable total knee replacements were introduced in the early eighties superseding an array of earlier designs, which were hampered with a high failure rate (Insall & Clarke 2006). Like in partial knee replacements, the distal femur and proximal tibia are replaced with metal components mostly made of cobalt chrome. In order to reduce friction a polyethylene insert is placed in-between the femoral and tibial components. Today's knee replacements follow two different design principles. In fixed bearing replacements, the polyethylene insert is rigidly attached to the tibial component, whilst mobile bearing replacements allow the insert to move somewhat independently between femur and tibia (Buechel et al 2001). Clinical studies have confirmed that TKR surgery is an extremely ‘position-sensitive’ operation, with mal-position or mal-alignment of the components invariably leading to poor results (Moreland 1988). Clinical outcome studies of contemporary total knee replacement designs show survival rates of up to 98% at 10 years and 95% at 15 years, which is equivalent to the survival of modern total hip replacements (Ranawat et al 1993, Ritter et al 1994, Pavone et al 2001).
MIS principles are somewhat different when applied to TKR surgery, as the patella remains in situ without being everted. This significantly restricts visibility and forcing the surgeon to perform component placement through either a small anterior surgical window or from the lateral or medial side (Fig. ). Critics hence continue to raise concerns MIS surgery can impede surgeon’s vision and may influence surgical accuracy and component alignment and possibly compromise long-term survival of the replacement (Fig.)(Dalury & Dennis 2005, Fisher et al 2003). Consequently a wider recommendation of MIS should only be expressed if long-term survival rates can equal those achieved with conventional knee arthroplasty.
Many surgeons have hence adopted a limited exposure of an otherwise traditional approach which reduces the incision length from 15” to around 8”to 10”, still maintaining some of the advantages associated with MIS surgery (Vaughan 2003). In some patients however, longer incisions are still necessary, especially if the pre-operative range of movement is significantly decreased or if a significant deformity (bowing/knocking) is present. Hence the surgeon has to assess the knee prior to surgery to judge upon the patient’s suitability for MIS. It is generally agreed that patients with a BMI > 40 should be excluded from MIS (Foran et al 2004, Tenholder et al 2005).
5. MIS IN COMBINATION WITH COMPUTER NAVIGATION
In order to overcome potential problems with MIS, computer navigation or image guidance has been introduced as aids to safeguard component alignment (Tria 2006). Due to the requirement of ‘surface mapping’ a wide exposure is still required. Experimental investigations however, have shown that computer-navigation used in conjunction with MIS may allow the accuracy with which the procedures are conventionally performed to be retained (Stuhlberg 2005, Song et al 2006). Computer navigated surgery should nevertheless not be seen as a panacea, and care should be taken not to rely entirely on computer systems without reflecting on recommended cutting angles as alignment errors are still possible (Fig. ). Hence surgeons should still be required to be familiar with the principles of traditional surgical technique. Although computer navigated surgery has been validated in improving mechanical alignment of the lower limb after TKR surgery and to improve reproducibility of this outcome measure, long term survival studies to assess an advantage to conventional TKR surgery are not yet available (Berry 2004).
6. DISCUSSION
The MIS procedure requires the surgeon to change the knee position during bone preparation and implant placement in order to move relevant anatomical structures into the surgical soft-tissue window. The reduction in soft tissue trauma is believed to decrease blood loss and pain, allowing for enhanced post-operative functional recovery (Bonutti 2004, Price et al 2001). Lower infection rates have been associated with MIS surgery, but conclusive clinical evidence is still missing. Despite these advantages patients receiving traditional knee arthroplasty appear to reach equal function levels at 3 to 6 months post surgery (Price et al 2001, Dalury & Dennis 2005).
Early equipment provided by manufacturers for the implantation of knee replacements were ill-designed if used in combination with MIS, and free-hand preparation of bone surfaces was often necessary jeopardising surgical precision. The major breakthrough came with the introduction of purpose designed MIS instrumentation and jigs which allowed for more reliable implant positioning (Fig ) (Laskin 2004).
The outcome of minimally invasive TKR is somewhat difficult to evaluate due to the variety of available surgical exposures currently in use, including mid-vastus, lateral, medial and sub-vastus to name only a few. Thereupon no validated standards concerning measurements of incision length or soft-tissue trauma are in existence. Measurements of surgical incisions are conveniently made in leg extension when the incision is up to 25% shorter then in flexion. The operation however is performed with the knee close to full flexion which increases the incision length considerably (Roidis et al 2007).
Surgeons who are interested in considering MIS should be aware of the substantial learning curve attached to this technique. In a recent publication King et al. reported on a single surgeon’s experience in familiarising himself with MIS (King et al 2007). It took almost 50 procedures before component alignment and operating time reached a standard comparable with those achieved by using a traditional approach. The researchers expressed concerns that the technique may not be suitable for the low volume arthroplasty surgeon (≤ 10 procedures per year) as the learning curve could potentially stretch out over several years. It is henceforth not surprising that ethical issues have been raised in connection with MIS (Holt et al 2006).
Holt et al. have expressed the view that ‘a number of potential disadvantages have been reported…which relate to the difficulty in performing such surgery within the restricted visual field and are compounded by the issues related to learning a new exposure and technique’. Concerns about potential problems with nerve and soft tissue injuries through excessive retraction, damage to prosthetic bearing surfaces at implantation through a small surgical window, and implant mal-positioning have been raised. With prolonged surgery and tourniquet times particularly in the beginning of the learning curve, associated complications such as thromboembolic events and infection may also be increased. In the hands of the experienced surgeon however, properly executed MIS provides definitive short term benefits whether these however warrant potential risks and uncertain long term out-come remain to be investigated.
In the meantime the use of a so called ‘limited exposure’ for both UKR and TKR surgery appeals to many orthopaedic surgeons as a well-come compromise (Vaughan 2003). It represents an adaptation of the conventional surgical technique rather than requiring the use of a completely different approach. Standard or MIS instruments may be utilised and through manoevering of the knee joint during surgery various aspects of the joint can be brought into the surgical window allowing for safe placement of the implant.
It is important that we maintain sight of the ultimate goal to safely improving patient’s quality of life with a procedure that must offer outcome measures that are at least as good as those achieved with conventional knee replacement surgery. Overall implant alignment should under no circumstances be compromised by reduced visibility through an inappropriately small incision. The potential advantages offered by MIS must be evaluated in a responsible manner, with appropriately designed and performed studies and peer reviewed publications before conclusions can be drawn and recommendations be expressed concerning the wider application of this new technique. Minimally invasive TKR surgery in particular should at present still be regarded as research and consented to as such. Thereupon patients should understand that they might not experience all of the benefits of this new procedure whilst the surgeon is still in the learning phase. Or as one orthopaedic surgeon put’s it ‘why trying to make an easy and reliable operation difficult’!