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Year : 2006  |  Volume : 40  |  Issue : 4  |  Page : 219-223
Hybrid fixation in rotating platform mobile bearing total knee arthroplasty using low contact stress knee

Department of Orthopaedics, Sir Ganga Ram Hospital, New Delhi, India

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Background : Mobile bearings in total knee arthroplasty reduce wear of polyethelene and mechanical loosening of components.
Method : Non-cemented Low Contact Stress (LCS) femoral implant and cemented LCS rotating platform tibial tray and polyethylene insert was used in 100 knees. Pre-operative diagnosis was osteoarthritis (OA) in 85 and rheumatoid arthritis (RA) in 15 knees.
Results : Ninetyeight knees were available for follow-up at a mean interval of 38 months. Minimum of 90° flexion was achieved in all the cases. All patients were females with a mean age of 67 years. Non progressive radiolucent lines of 1-2mm were seen in 6 femoral components. Twelve tibial components also showed similar radiolucent lines in 2 or less zones. Knee Society Knee Score improved from 32 to 93 and function score from 47 to 79. Results were good to excellent in 95 knees. There was no case for bearing spin out or revision.
Conclusion : Results of this study suggest that the hybrid fixation can reliably provide excellent or good results in selected patients. Key-words: Mobile bearing knee; Arthroplasty; Porocoat; Hybrid.

How to cite this article:
Maini P S, Kailey P V, Talwar N. Hybrid fixation in rotating platform mobile bearing total knee arthroplasty using low contact stress knee. Indian J Orthop 2006;40:219-23

How to cite this URL:
Maini P S, Kailey P V, Talwar N. Hybrid fixation in rotating platform mobile bearing total knee arthroplasty using low contact stress knee. Indian J Orthop [serial online] 2006 [cited 2020 Feb 27];40:219-23. Available from:

   Introduction Top

Knee is a complex joint and its degeneration in age related post-traumatic and rheumatoid like arthropathies may give rise to severe disability. Modern total knee replacement prostheses and the operation have evolved over many decades. The procedure has proved to be successful in relieving pain, correcting deformity and providing functional range of movement and is now an established operation. Despite these improvements, greater demands have been placed on these implants as a result of improved design kinematics and extension of indications for TKA to include younger, more active patients. In addition, concern about the degradation of the mechanical properties of PMMA, the potential of third body wear from PMMA debris, and deterioration of the bone cement interface has stimulated research for a more desirable method of implant fixation than PMMA. Clinical studies regarding the early results of uncemented TKA are limited and conclusions variable[1],[2],[3].

The quality of fixation of non-cemented tibial component has been questioned. Retrieval studies of uncemented tibial components have demonstrated that very little bone ingrowth actually occurs into the implants, challenging the long term durability of this method of fixation[4],[5],[6].

The introduction of mobile bearings in total knee arthroplasty has attempted to resolve two of the most important challenges, namely, wear of polyethelene and mechanical loosening of components. These bearings have congruity to overcome the problem of high contact stresses causing wear and are mobile to eliminate constraint forces and so to reduce mechanical loosening.

The present study was undertaken to evaluate the short­term results of the unique concept of mobile bearings and hybrid fixation.

   Materials and methods Top

One hundred consecutive hybrid TKAs in 54 patients were observed prospectively and according to the clinical and roentgenographic guidelines of the knee society[7].

The low contact stresses prosthesis (DePuy), with an uncemented, porous ingrowth porocoat Co-Cr femoral component and a cemented Co-Cr tibial component was used in all patients. Design features of the femoral component include the use of a Co-Cr porocoat porous ingrowth surface and sacrifice of the PCL. The tibial component design includes a central stem with textured surface. It has rotating platform using polyethylene insert, which rotates inside the stem of the tibial tray.

The present series comprised of only female patients because of the fact that LCS Sm+ (Small Plus) femoral component is available only as porocoat variety intended for non-cemented use. Most of the Indian female patients' knees accepts only SM+ femoral component hence this unusual situation of using this modality in females only. Average age at the time of surgery was 67 years (52 - 86 years). The patient's diagnosis was OA in 85 and RA in 15. Twelve knees in 11 patients had a total of 12 surgical procedures performed before TKA. These consisted of four high tibial osteotomies and 8 meniscectomies.

Eighty six knees were in varus alignment pre-operatively (0°-20°) and 14 knees were in valgus alignment pre­operatively average 8° (range 2°-15°). Twenty two knees had a pre-operative flexion contracture average 8° (range 5°-25°).

Selection of this technique of implant fixation was based primarily on the clinical judgement of the surgeon and taking into consideration patient's age, medical condition, activity level and intra-operative assessment of bone quality. In patients who might otherwise be candidates for uncemented TKA, proximal tibial bone quality may be insufficient to allow insertion of an uncemented tibial component, despite the presence of adequate femoral bone stock. These patients frequently are candidates for hybrid implant fixation. Patients with significant osteopenia or who have medical conditions that might adversely affect bone growth, or who have some serious medical condition affecting longevity or are relatively inactive are typically candidates for cemented TKA.

In addition to assessment of bone quality, several other conditions evaluated at the time of surgery were considered pre-requisites to the use of the hybrid technique. Because LCS is a minimally constrained cruciate sacrificing knee, evaluation of the ligament competency and accurate balancing of the ligament was a necessity. Because bone ingrowth fixation required close apposition of cancellous bone to the porous implant surface and secondary press fit prosthesis stability to limit interface micromotion, accurate femoral preparation is mandatory when using this technique.

A standard post-operative care and rehabilitation protocol was used for all patients. Patients were encouraged for early ambulation with full weight bearing with knee brace and walker support. Active range of motion exercises were started on 3rd or 4th post-operative day and continued till they achieved 90° flexion and then discharged. In all patients prophylaxis for thromboembolic disease consisted of low molecular weight heparin in standard doses.

All patients were evaluated at regular interval of 2 weeks, 4 weeks, 2 months, 3 months, 6 months and yearly thereafter. Follow-up interval for these patients averaged 38 months. 2 patients were lost to follow-up. Clinical and radiological examination, in accordance with recommendations of the Knee Society[7] was obtained pre-operatively and at regular interval post-operatively for all patients. Standard pre-operative and post-operative roentgenograms, consisting of anteroposterior (AP), lateral and skyline view for patellofemoral joints, were used to determine knee alignment, component position and alignment, joint line position, patellar thickness, patella height, patella tilt and the presence of subsidence or radiolucent lines at the bone growth or bone cement interface. Forty eight knees had patellar resurfacing while the rest had patelloplasty. Zonal analyses of the interface radiolucent lines were also done in accordance with the guidelines of Knee Society. In addition, fluoroscopically guided AP and lateral X-rays were obtained, where indicated, to further assess the tibial and femoral interfaces.

   Results Top

The average pre-operative knee society knee score was 30 (range 18-53); the average functional score 43 (range 0-60) and the average pain score 15 (range 0-20). At the time of last follow-up examination, the average knee society knee score was 92 (range 55-99) and the average pain score was 47 (range 10-50), the average functional score improved to 80 (range 50-100).

Eighty two knees had a knee score of 90-100, 12 knees of 80-89, 3 knees of 70-79 and one knee had a knee score of less than 60 at last follow-up examination. Ninety knees had either absent or mild pain only. All patients were able to ascend and descend stairs post-operatively. Range of motion averaged 100° pre-operatively (range 70-130°) and 112° post­operatively (range 80-125°). More than 90% patients obtained flexion of more than 100°. Only two patients had flexion contracture post-op, measuring 5°. No knees had any clinically significant AP or mediolateral instability post-operatively.

X-Ray evaluation: The femur, tibia and patella interfaces were divided into zones, with each zone evaluated for the presence of radiolucent lines, and if present, their size and distribution [Figure - 1] a,b and [Figure - 2]a,b. As recommended by the Knee Society, the zone locations used were established by the developers of the implant. In addition to width of the lucencies, the extent of the lucencies in each zone (incomplete or complete) was also recorded.

Eighty six knees had no radiolucent lines at the bone cement interface on the AP view of tibia. Twenty knees had incomplete, non-progressive lucencies in a single peripheral zone only. Eight of these were less than 1mm and 2 were less than 2mm in width. No lucencies were noted centrally or around the tibial stem in any knees.

Evaluation of the lateral X-ray of the distal femur revealed no radiolucent lines at the bone prosthetic interface in 92 knees. Four knees had incomplete lucencies less than 1mm wide in one or more zones. Isolated incomplete lucencies in zone 2 & 3 occasionally in 2 other knees. All these measured less than 1mm. Seven knees revealed some adverse bone remodelling consistent with stress shielding under the femoral component in the form of localized area of osteoporosis. Radiolucent lines were absent at the bone-cement interface in 43 of the patellar implants (of the 46 patellae that were replaced). Three knees showed lucencies, which were incomplete, less than 1mm wide and located at the inferior or superior poles.

Complications: Three knees had minor wound-healing problems, which did not compromise excellent clinical and functional results. All knees were managed by dressing changes and oral antibiotic administration. There were no deep infections post-operatively.

Three knees developed superficial phlebitis and one developed deep vein thrombosis that was managed with routine anticoagulation therapy. There were no symptomatic pulmonary emboli identified in any patients.

Two knees developed mild anterior knee pain post­ operatively, although no apparent reason could be found out. Despite this complication, in both the patients the functional result was good. At the time of last follow-up examination their knee score was 76 and considered only fair because of this mild persistent pain.

One knee, a 68 years old woman, who had hybrid TKA could not achieve knee flexion more than 85°. Although her pain relief and functional results have been very satisfactory (pain score 45 and fixation score 70), her knee society score was considered only a fair result. None of the patients have shown any symptom of instability, loosening, abnormal wear or mal-alignment. No components of any of the patients in this study have been revised for any reason. There have been no incidents of bearing spinout.

   Discussion Top

With advances in the understanding of biomaterials, knee biomechanics and knee kinematics modern knee arthroplasty has become a reliable and durable method of treatment for painful arthritis of knee. With earlier implant designs, tibial component loosening was by far the most common cause of knee failure with cemented fixation[8].

Certain design of the tibial component made to incorporate metal bearing and improved coverage of the tibial surface by the implant greatly reduced the incidence of tibial loosening[9],[10].

As a result, the indications for TKA have been extended to younger and more active patients. Concern about the degradation of the mechanical properties of PMMA, the potential for third body wear from PMMA debris, and deterioration of the bone cement interface has stimulated the search for alternate methods of implant fixation for use in these situations. Thus, cementless porus coated total knee prosthesis became a popular alternative to cemented fixation with PMMA. The early clinical results of uncemented TKA have been variable.

Dodd et al[1] compared clinical and radiological results of 18 patients with cemented PCA (Porous coated anatomic Howmedica, Rutherford NJ) knee arthroplasty of one knee and uncemented PCA arthroplasty of the contralateral knee. At an average follow-up of more than 5 years, no difference in pain score, function or patient preference was apparent between the two. Similar results have been reported with uncemented prosthesis in patients younger than 50 years of age[11].

Stulberg et al[12] compared several variations of cemented and uncemented component fixation and concluded that although reliable ingrowth fixation of uncemented femoral component occurs stable ingrowth fixation of tibial component is less predictable.

Retrieval analysis has shown that bone ingrowth into uncemented porous tibial component is limited. Clinical problems with uncemented tibial component loosening and pain and the variable bone ingrowth into uncemented tibial component has raised concern about the durability of uncemented tibial component fixation. The reported success of uncemented femoral component fixation and the cemented tibial component fixation have been incorporated into the concept of hybrid TKA by the authors.

Results of our present study show this technique to be a reliable method of fixation over an average follow-up period of 60 months. Pain relief and fixation was good or excellent in up to 90% of patients in this study and these results have not shown any signs of deterioration. Clinical and radiological evaluation of femoral component loosening were absent in all our patients. Some patients showed interface lucencies under the femoral component. These were, however, limitations in distribution and magnitude with many of them the result of initial incongruities between the cut surface of the anterior femur and the implant. Further follow-up evaluation is necessary to establish the long-term effects of uncemented femoral component fixation on the bone remodeling in the distal femur.

Unlike uncemented tibial component, stable biological fixation of uncemented femoral component appears to occur predictably in all but the most osteopenic patients. The relative superior bone quality of the distal femur, the intrinsic press fit stability of the femoral component, and the different loading pattern of the distal femur may be responsible for this phenomenon. Although these early results are promising determination of the ultimate durability of this biologic fixation will require long-term follow-up analysis.

The potential advantages of uncemented femoral component fixation include decreased operation time and reduction of poly wear from PMMA debris. It has the potential of providing life long fixation. In properly selected patients, hybrid TKA can allow these potential benefits to be realized without compromising the results obtainable with a cemented TKA.

Although described as one of the most successful surgeries of the last century, issues of polyethylene wear and mechanical loosening of the components have proved to be very troublesome and detrimental to the long-term survivorship. Mobile bearing arthroplasty has attempted to resolve these issues to quite an extent. They did so due to the fact that these knees were congruous, which overcame the problems of high contact stresses causing wear and they were mobile, so that constraint forces could be eliminated, hence reducing mechanical loosening.

In the early period of total knee arthroplasty, the issue of wear was not considered a serious problem. The design of the prosthesis varied from being fully congruent to highly incongruent ones. While fully congruent joints produce unacceptably high constraint forces, thereby allowing the shear forces to be transmitted to bone prosthesis interface, thus causing early loosening. Less congruent joints, on the other hand, removed these constraints and produced increased movements, but because of reduced area of contact between the metal and polyethylene produced increased wear of the polyethylene, which ultimately lead to osteolysis.

The concept of the rotating platform mobile bearing knee evolved from the need to create a better knee arthroplasty system. A kinematic conflict occurs in the conventional knee prosthesis between increasing conformity and rotational freedom. The mobile bearing knees resolve this conflict, thus creating a situation of high contact area, but reduced contact stresses. Experimental studies have shown that highly congruent surfaces undergoing unidirectional articulation have reduced wear rates because of what is known as "Strain Hardened Effect". Rotating platforms with flat poly bearing surfaces on a highly polished Cobalt Chromium tibial base plate diminished the "backside wear" of polyethylene. The results of the rotating platform total knee arthroplasty compare favourably with results found with fixed bearing total knee systems.

In summary, we would like to point out that we have used the concept of mobile bearing knee for its advantages over fixed bearing, and, in addition, used the method of hybrid fixation with its advantages over the cemented one. To the best of our knowledge, there is no published study of a hybrid knee using the mobile bearing concept.

   References Top

1.Dodd CAF, Hungerford DS, Krackow KA. Total knee arthroplasty fixation comparison of the early results of cemented v/s uncemented porous coated anatomic knee prosthesis. Clin Orthop. 1990; 260: 66.  Back to cited text no. 1    
2.Hungerford DS, Kenna RV. Preliminary experience with a knee pros­thesis with porous coat without cement. Clin Orthop. 1983;178: 95.  Back to cited text no. 2    
3.Landon GC, Galante JO, Meley MM. Non-cemented total knee arthro­plasty. Clin Orthop. 1986; 205: 49.  Back to cited text no. 3    
4.Cook SD, Thomas KA, Haddad RJ. Histologic analysis of retrieved human porous coated total joint components. Clin Orthop. 234: 90; 1988.  Back to cited text no. 4    
5.Haddad RJ, Cook SD, Thomas KA. Biologic fixation of porous coated implants. J Bone Joint Surg (Am). 1987; 69: 1459.  Back to cited text no. 5    
6.King TV, Scott RD. Femoral component loosening in TKA. Clin Orthop. 1985;195: 285.  Back to cited text no. 6    
7.Ewald FC. The knee society total knee arthroplasty roentgenographic evaluation and scoring system. Clin Orthop. 1989; 248: 9.  Back to cited text no. 7    
8.Scuderi GR, Insall JN, Windsor RE, Moran MC. Survivorship of cemented knee replacements. J Bone Joint Surg (Br). 1989; 71: 798.  Back to cited text no. 8    
9.Bartel DL, Bursteen AH, Santavicca EA, Joly L. Performance of the tibial component in total knee replacement. J Bone Joint Surg (Am). 1982;64: 1026.  Back to cited text no. 9    
10.Lewis JL, Ashew MJ, Jayrox DP. A comparative evaluation of tibial component designs of total knee prosthesis. J Bone Joint Surg (Am). 1982; 64: 129.  Back to cited text no. 10    
11.Hungerford DS, Krackow KA, Kenna RV. Cementless total knee replacement in patients 50 years old & under. Orthop Clin North Am. 1989; 20: 131.  Back to cited text no. 11    
12.Stulberg SD, Stulberg BN. The biological response to uncemented total knee replacements. Orthop Trans. 1986;10: 169.  Back to cited text no. 12    

Correspondence Address:
P V Kailey
16B, DDA Flats, Gulabi Bagh, New Delhi – 110 007
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0019-5413.34498

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