34Specific Issues in Surgical Techniquesfor Mobile-Bearing Designs 34 34

Chapter 34 · Specific Issues in Surgical Techniques for Mobile-Bearing Designs – P. T. Myers

34 Specific Issues in Surgical Techniques for Mobile-Bearing Designs

P. T. Myers

Summary

Mobile-bearing knee arthroplasty promises better kine- matics, improved range of motion, and implant longevi- ty.Various designs have evolved since the procedure was first introduced in the late 1970s. Successful mobile-bear- ing arthroplasty relies on obtaining movement as closely as possible around the original mechanical axes and with- in the normal soft-tissue envelope of the knee. Retaining tension of the collateral and posterior cruciate ligaments is essential, along with preserving the joint line. If these are not achieved then it is unlikely that the mobile com- ponent will move, thus compromising the result.

Introduction

Mobile-bearing knee arthroplasty (MBKA) involves having a mobile polyethylene (PE) insert between the femoral component and the tibial tray (

). The concept was developed with the intention of more close- ly mimicking the kinematics of the intact knee, improv- ing range of motion, and reducing point loading of the PE component in the hope of reducing wear and im-

proving longevity [1]. To date, none of these goals have been definitively achieved by any of the MBKA models available. There is also a recognized potential risk of in- creased backside wear [2, 3]. However, it remains the de- sire of surgeons to achieve the best results for their pa- tients, and in searching for the optimal knee replace- ment, there will be continued use and development of the MBKA. While there is no strong evidence indicating that MBKA is significantly better than fixed-bearing knees, there is similarly no evidence to the contrary. In- deed, studies have shown that a mobile-bearing (MB) implant is more tolerant of technical errors of malalign- ment and does show less point loading in flexion in some models [4-6]. Certainly, there is sufficient evi- dence to support continued use and development of the MBKA [6]. While the surgical principles are similar for the implantation of fixed and MB components, some as- pects relating to rotation, alignment, and tension are im- portant and worth considering separately in the context of a MBKA.

Types of Mobile-bearing Knee Replacements

The various types of MBKA available can be categorized according to the PE movement allowed and the function- ing of the posterior cruciate ligament (PCL):

1. Polyethylene movement allowed by the implant – Rotation only

– Translation only – Rotation and translation – No constraints

2. Posterior cruciate ligament – Released

– Released and stabilized (by the implant) – Retained

The mechanisms restraining the movement of the PE component, by posts, rotating stems, grooves, rails, or others,all imply some opportunity for increased PE wear.

This potential complication may prove to be a problem in the future.

⊡ Fig. 34-1a, b. AP(a) and lateral (b) radiographs of a mobile-bearing knee arthroplasty

a b

Those implants which allow more freedom of movement of the PE component will require the knee itself to be more stable. If the mobile component has few or even no limitations on its motion and the PCL has been released, then the component may move more than is required or even dislocate (see

). Excessive movement of a PE component can result in symptomatic clicking with movement. These aspects must be considered at the time of surgery.

Surgical Issues to Be Considered with MBKA Designs

Space for Movement of the Implant

At the time of implantation of the trial components and, more importantly,of the definitive implant,there must be a clear space for the movement of the mobile bearing. If the PE is required to rotate or translate beyond the mar- gins of the tibial component,then this should be possible.

Clearance of soft tissues and bone at the margins of the implant should be achieved with the surgical exposure but must be checked at the time of implantation. Compo- nents that allow anterior and posterior translation should be shaped so as not to impinge anteriorly on the fat pad and the overlying patella tendon, or posteriorly on the PCL. It may be necessary to excise some of the patella fat pad to allow full anterior movement of the PE in exten- sion. Patients may feel anterior discomfort in full exten- sion with some implants that are more bulky and impinge on the anterior soft tissues as they translate in terminal extension.

The constraint on the MB component should be within the parameters that will allow it to move and yet remain stable. Excessive constraint will diminish range of mo- tion. Conversely, laxity across the MB component may result in symptoms ranging from a clunking sensation with movement to frank instability and even dislocation of the implant.An implant that allows translation requires some block to posterior movement of the tibia to prevent the implant from dislocation. The PCL provides the nat- ural resistance to this motion, and so it should ideally be left intact [7]. Otherwise, the MB component must have some form of block on the tibia (a post or an anterior stop) and a limiting mechanism on the femoral side (deep dish articulation or femoral post).Aspects of ligament balanc- ing in a MBKA will be dealt with later in this chapter.

The Importance of Joint Line Position

Optimal functioning of any knee arthroplasty relies on the interaction between the geometry of the implant and the soft-tissue restraints. In order that the soft tissues can simultaneously allow movement and control stability, the prosthesis must have a similar contour and move around axes similar to those of the native joint. Hence, the posi- tion of the joint line in the replaced knee must be as close as possible to the original position in both flexion and extension in order to allow normal soft-tissue tension and stability [7, 8].

Much of the controversy, difficulty, and clinical prob- lems surrounding knee arthroplasty could be avoided if more attention were paid to the joint line position. This applies equally to fixed and MBKA [8]. The majority of patients undergoing knee arthroplasty do not have severe ligament contractures or deficiency, and few have gross deformity. Consequently, the need for “ligament balanc- ing” should be uncommon, or at least restricted only to more difficult cases. The perceived need to balance liga- ment tension and equalize flexion and extension gaps is largely the result of ignoring the principles of restoring original joint geometry and joint line position. If these fundamental principles are followed, then the complicat- ing issues tend not to arise. For example, if in a knee with minimal or no medial bone loss the tibial cut is made just below the medial tibial surface, removing, say, 2 mm of bone, and the minimal tibial component thickness is 10 mm then the joint line has effectively been raised by 8 mm. There are two possible methods to accommodate this error; either the distal femoral cut must take more bone, thus also forcing the posterior condyles to be cut short to allow “equal flexion and extension gaps”, or, if correct femoral bone cuts are made, then the joint will be too tight and “ligament balancing by releases”will have to

34

⊡ Fig. 34-2.Lateral view showing anterior dislocation of mobile- bearing component

be undertaken (see

). Neither of these would have been necessary if the joint line had been correctly placed initially.

Achieving Range of Motion

Extension.

For an MBKA to achieve full extension the fol- lowing operative factors need to be addressed.

▬ Adequate bone cuts should be made to allow full ex- tension, and the PCL must not be tight, particularly if there has been a flexion deformity preoperatively.

▬ There must be no obstruction to anterior movement of the mobile component (see above).

▬ The extensor mechanism must function correctly, re- quiring:

– Sufficient length and strength of the muscle/tendon unit to achieve full extension

– Adequate strength of the quadriceps repair – Stability of the patella

– Adequate lever arm effect of the patella tendon, i.e., the joint line must not be so high as to reduce the mechanical advantage of the patella.

▬ Posterior osteophytes, if present, should be removed.

▬ Occasionally, the posterior capsule will need to be re- leased to allow full extension.

Flexion.

The factors that can assist in gaining optimal flex- ion include:

▬ Allowing the MB component to glide and/or rotate as required to allow for the altered kinematics resulting from deficiency of the anterior cruciate ligament

▬ Restoring the posterior tilt of the tibia and possibly increasing it by 1°-2°

▬ Restoring the length of the posterior femoral condyles. This is also known as the posterior condyle offset [9] (see

).

▬ Resecting the posterior osteophytes of the femoral condyles and shortening the posterior condyles to the same size as the prosthesis. (This is required because the sagittal radius of the posterior condyles of the prosthesis is usually smaller than that of the normal knee; see Fig. 34-4.)

▬ Ensuring the PCL is not excessively tight

▬ Maintaining the joint line at the correct height

Rotation.

Correct rotational alignment of the implants in the longitudinal axis is paramount to the success of any knee arthroplasty. While the MB component may “self- correct” any small errors of rotation of the tibial compo- nent,it may be compromised if there are rotational errors in positioning of the femoral component.

Aspects affected by incorrect rotation of the femoral component include:

▬ Patella position: Excessive internal rotation may re- quire an extensive lateral release; the patella may dislocate; excessive tension may lead to pain or restricted flexion. External rotation may cause the patella to be too loose, predisposing to subluxation or dislocation.

▬ The collateral ligaments will be either too tight or too loose (depending on the direction of the rotational er- ror), leading to a number of potential problems such as:

– Varus or valgus malalignment or instability in flex- ion

Chapter 34 · Specific Issues in Surgical Techniques for Mobile-Bearing Designs – P. T. Myers

⊡ Fig. 34-3.Lateral radiograph showing a raised joint line as evidenced by the height above the fibular head and the level of the patella

⊡ Fig. 34-4.The posterior condyle offset (A) should be restored as close- ly as possible by the implant. The line for removal of posterior bone (B) should include the remaining proximal portion of the posterior condyle as well as the posterior osteophytes

– Dislocation of the MB component

– Excessive contact pressures predisposing to PE wear – A perceived need to “balance” the ligaments by re- leasing the “tight” side, thus creating an unstable joint

Incorrect rotation to a minor degree of the tibial compo- nent can be accommodated by a rotating MB component, and this is probably one of the perceived advantages of the MBKA [5]. However, there are two situations where tibial implant rotation errors can be a problem: (a) Excessive rotation of the tibial base plate may result in the mobile PE component overhanging the margins of the base plate, leading to a risk of dislocation and/or point loading and possible fracture of the component. (b) Malrotation of the tibial base plate can change the effective varus-valgus alignment of the tibia if there is anteroposterior tilt in- corporated into the cut. This can be difficult to appreci- ate at the time of surgery (see

).

Author’s Preference Exposure

A tourniquet is used (without exsanguination) and a mid- line skin incision is deepened to the retinaculum. The quadriceps tendon is split longitudinally,with a small me- dial margin left for repair and a very definite angle made at the superomedial corner of the patella to ensure accu- rate reattachment. The parapatellar retinacular and cap-

sular incision extends medial to the tibial tuberosity and the medial periosteum is reflected. The menisco-tibial at- tachments are released to the posterior aspect of the tibia.

The fat pad is released from the tibia and the lateral menis- cus and the lateral capsule are dissected from the tibia and reflected laterally. The patella is dislocated and everted.

The knee is flexed and the soft tissues are released as re- quired to obtain full flexion. The fat pad is not resected.

Bony Preparation

Rotational alignment is important to minimize liftoff in flexion and possible spinout of the MB. Referencing from the posterior condyles is most common but is inaccurate if the articular cartilage has been eroded or in the valgus knee. Aligning with the epicondyles is ideal but intraop- erative identification is often not accurate. This is even more difficult with mini-incision techniques.Whiteside’s line is perpendicular to the epicondylar axis in 97% of cases [10]. Osteophytes are removed and the appropriate jigs are used to prepare the distal femur. Computer navigation systems are becoming more advanced and accurate but are not in general use. The primary benefit of these systems is that they force the surgeon to look very closely at the bony architecture and to constantly check and recheck alignment and position of bony cuts.

In most implant designs the length of the prosthesis posterior condyles is less than the length of the native condyles. This is due to the reduced posterior sagittal ra-

34

⊡ Fig. 34-5a-h.Correct rotational alignment of the tibial cutting block produces a neutral varus/valgus alignment with a posterior tibial slope (a-d).

However, incorrect rotational alignment of the tibial jig with posterior tilt will cause a varus or valgus malalignment (e-h)

a b c d

e f g h

dius. The bony condyle plus any posterior osteophytes must be removed to allow the PE to move around the femoral component, thus maximizing flexion. It is important that the condyle length be maintained, as this determines the flexion gap and the capacity for flexion (see Fig. 34-4).

In cases with a long-standing fixed-flexion deformity posterior capsular release may be required, and this is usually done by releasing proximal to the posterior condyles after resection of the osteophytes.

Tibial alignment and rotational positioning is the most difficult step of knee arthroplasty surgery. For im- plants which require a posterior tibial tilt (usually 3°) it is imperative that the alignment and rotation be precise.

Any error of rotational alignment of the tibial cut by virtue of the tibial tilt causes an alteration of the varus/valgus alignment. A mobile bearing may self-cor- rect for rotational malposition; however,the altered varus or valgus alignment results in a widened flexion gap in one compartment which could lead to dislocation of the PE component (see Fig. 34-5). An external alignment tib- ial jig is preferred for varus-valgus determination because the medullary canal of the tibia is often curved [11]. The amount of bone to be resected must be measured from the more normal side and should equal the thickness of the tibial component. This ensures that the original joint line is maintained. Ligament tension in flexion and ex- tension should be checked before the jig is completely re- moved. Close attention must be paid to the alignment of the tibial cut and it must be checked before implantation of the trial tibial component.

Posterior Cruciate Ligament

Most MBKA designs retain the PCL [12] as a means of controlling the constraint on the PE and thus (theoreti- cally) allowing it to move. If the PCL is sacrificed then the anteroposterior stability of the joint is controlled only by the PE constraint on the tibia.If the PE is to translate,then the PCL must remain intact. To reflect the PCL, the tibia is levered anteriorly and an osteotome is used to careful- ly reflect an osteoperiosteal flap, preserving the original lower portion of the PCL origin. If the PCL is subse- quently tight in flexion then partial release from the fe- mur is all that is required.

Patella

It is the author’s preference not to routinely resurface the patella. In cases where there is articular cartilage degen- eration to bone in the patellofemoral joint, a patellar but- ton is cemented in place. Otherwise, the patella is just débrided of osteophytes.

Trial Implantation

The trial components are then inserted, and stability and range of motion are checked. Aspects which are specifi- cally checked include:

1. In extension:

– That full extension is obtained – Position of the joint line

– Varus-valgus alignment of the knee – Stability

2. In flexion:

– That adequate flexion is obtained

– That the patella is tracking normally and is stable – Stability

– Position of the joint line. Determining the position of the meniscal rim in relation to the prosthesis contact surface best assesses this (see

).

– There should be no liftoff of the PE insert which could be a result of inadequate posterior condyle resection. (Other potential causes of PE liftoff such as an inadequate flexion gap or a tight PCL were as- sessed earlier at the time of the tibial resection.) – The PE insert is mobile with no soft-tissue im-

pingement. At this stage some of the fat pad may need to be removed.

Implantation and Closure

The joint is then thoroughly irrigated and the definitive implants are inserted, usually uncemented. The wound is closed in layers over a reinfusion drain system. A simple extension knee splint is applied in recovery, and the pa- tient begins rehabilitation within 24 h, with passive flex- ion to 90°. The splint is removed when straight leg raising is accomplished.

Chapter 34 · Specific Issues in Surgical Techniques for Mobile-Bearing Designs – P. T. Myers

⊡ Fig. 34-6.Operative photograph showing that the joint line is maintained in flexion. The meniscal rim (black arrow) is adjacent to the prosthesis contact surface (white arrow)

Future studies will determine whether MBKA is ulti- mately more beneficial to the patient than a fixed-bear- ing implant. Subjective issues of patient satisfaction and function are paramount, although factors such as actual movement, component stresses, and wear characteristics need to be further assessed.Ultimately,surgeons will con- sider these various issues and will also be influenced by the ease and accuracy of implantation of the device.

In general terms, the principles of performing MBKA are the same as those for a fixed-bearing arthroplasty.

However, attention to precise positioning is required for the MB component to be able to move as designed.

Whether this translates to a better functional result for the patient as a result of the technique or the bearing mo- bility is yet to be determined. It is evident that MBKA has a place in the management of degenerate knee conditions, as no major problems have been reported in the past 30 years.Research and development should continue to pro- duce better kinematic knee designs, and hopefully the potential benefits of MBKA will be realized.

Knee arthroplasty has progressed exponentially in the past 50 years. The future promises improvements in design, materials, fixation, and lubrication. Genetic and biological technologies may have a role in future knee replacement surgery. The challenge will be whether to remove otherwise normal structures and have their func- tion replaced by the prosthesis,or to retain as much of the normal knee as possible and have the prosthesis function within that framework.

Acknowledgements.

I would like to thank Mr. Adrian Wilson, M.B.B.S., B.Sc., F.R.C.S., F.R.C.S. Orth, and Mr.

Mark Watts, B.Sc., App.H.M.S.(Ex-Sc) Hons, for their assistance with this chapter.

1. Menchetti PPM, Walker PS (1997) Mechanical evaluation of mobile bear- ing knees. Am J Knee Surg 10:73-82

2. Wasielewski RC (2002) The causes of insert backside wear in total knee arthroplasty. Clin Orthop 404:232-246

3. Chapman-Sheath PJ, Bruce WJ, et al (2003) In vitro assessment of proxi- mal polyethylene contact surface areas and stresses in mobile bearing knees. Med Eng Phys 25:437-443

4. Cheng CK, Huang CH et al (2003) The influence of surgical malalignment on the contact pressures of fixed and mobile bearing knee prostheses - a biomechanical study. Clin Biomech (Bristol, Avon). 18:231-236 5. Stukenborg-Colsman C, Ostermeier S, et al (2002) Tibiofemoral contact

stress after total knee arthroplasty: comparison of fixed and mobile-bear- ing inlay designs. Acta Orthop Scand 73:638-646

6. Price AJ, Rees JL, et al (2003) A mobile-bearing total knee prosthesis compared with a fixed-bearing prosthesis. A multicentre single-blind randomised controlled trial. J Bone Joint Surg [Br] 85:62-67

7. Morberg P, Chapman-Sheath P, et al (2002) The function of the posterior cruciate ligament in an anteroposterior-gliding rotating platform total knee arthroplasty: J Arthroplasty 17:484-489

8. Beverland D (2002) Advanced mobile-bearing surgical technique:

Orthopedics 25 [Suppl 2]:s265-271

9. Bellermans J, Banks S, et al (2002) Fluorscopic analysis of the kinematics of deep flexion in total knee arthroplasty. Influence of posterior condylar offset: J Bone Joint Surg [Br] 84:50-53

10. Whitesides LA, Arima J (1995) The anteoposterior axis for femoral rota- tional alignment in valgus total knee arthroplasty. Clin Orthop 321:168- 172

11. Tosun N, Aydinlioglu A, et al (2003) Anatomical characteristics of the tibial medullary canal and their implications for intreamedullary fixation.

J Int Med Res 31:557-660

12. Vertullo CJ, Easley, et al (2001) Mobile bearings in primary knee arthro- plasty. JAAOS 9:355-364

34