Knee Spacer
T. Villa, D. Carnelli, R. Pietrabissa
Laboratory of Biological Structure Mechanics, Department of Structural Engineering, Politecnico of Milan, Italy
Introduction
Infection is one of the most severe complications that can arise after the implant of a knee prosthesis [2, 7]. Late infections are usually treated by removing the infected prosthesis and reimplanting a new one, following either the one or two stages tech- nique: in order to avoid persistent and recurrent infections that have been reported when using the one-stage technique [8, 10] the two-stage technique has been intro- duced, consisting in the removal of the infected prosthesis in a first operation stage and in the reimplantation of a new one after adequate treatment of the infection in a second stage, usually performed six to twelve weeks after the first procedure [7, 10, 12, 14].
In the interim period the usage of a knee spacer is suggested in order to avoid patient immobilization and soft tissues contracture with consequent shortening of the operated leg and difficulty of reimplantation [11, 12]. In cases of knee spacers made of antibiotic addicted bone cement, a locally focused therapy to prevent coloni- zation [9] is achieved.
The knee spacer can be static or mobile, the difference laying in the possibility of knee flexion during the interim period: static spacers have shown difficulties in the exposure at the time of reimplantation due to quadriceps shortening and soft tissues adherences, a poor range of motion postoperatively, instability and bone erosion [4]:
these drawbacks moved the surgeons to the usage of mobile articulating spacers that have improved reimplantation and a better range of mobility of the knee after the sec- ond operation stage [5, 10, 13].
Considering that six to twelve weeks must pass between the two operating stages and that the patient is often allowed to bear weight in order to prevent soft tissue con- tracture, the assessment of the mechanical performances of mobile knee spacers made of polymethyl methacrylate (PMMA) bone cement is a key issue to evaluate their reliability before the clinical use.
In particular the loads that act on the spacer come from the walking activity of the patient that can be defined as a cyclic activity, requiring a repetition of a high number of cycles of load. The fatigue performances of the device must be investigated in order to prevent failures during the clinical usage.
In this work the fatigue performances of Tecres Spacer-K knee spacer were investi-
gated through two types of tests: (i) resistance tests on the whole device (condyles +
tibial plate) run on a four degree of freedom knee simulator basically following the
prescription of ISO 14243 standard; (ii) fatigue tests on the tibial tray run according to ASTM F 1800 – 04 standard.
Materials and Methods
The knee spacer that has been subjected to tests (Fig. 1) is made of a tibial component consisting in a flat base upon which a femoral component articulates, thus permitting the flexion of the knee. Both the components of the device are made of PMMA con- taining 2.5 % w/w gentamicin and are fixed to the bone using antibiotic addicted bone cement. The spacer is made in three different sizes (small-S, medium-M and large-L) and is manufactured by Tecres S.p.a., Sommacampagna (VR), Italy with the commer- cial name of Spacer-K.
All the tests have been performed at the Laboratory of Biological Structure Mechanics of the Politecnico di Milano, Milan, Italy.
Cyclic Tests on the Whole Spacer
Cyclic tests consisted in applying to the knee spacer 500,000 loading cycles in order to assess its mechanical resistance during a period of time corresponding to a six month walking activity, which could be the period of implant of the spacer.
The tests were carried out on three spacers of different size (small-S, medium-M and large-L) and were performed on a four degrees of freedom MTS knee simulator (MTS Systems, Minneapolis, MN, USA) (Fig. 2) mounted on a MTS Bionix 25kN- 250Nm axial-torsional testing machine. The knee simulator allows to impose simulta- neously with the axial force three kinematic conditions, namely the flexion-extension and the internal-external rotation of the femoral condyles and the antero-posterior (A-P) shear of the tibial plate: the axial force simulates the action of the bodyweight during walking on the knee while the three kinematic conditions are the basic com- ponents in which the multidirectional movement of the knee during the gait cycle can be divided.
Fig. 1. Components of the Tecres knee spacer
˜
Fig. 2. The MTS knee simulator
Fig. 3. Load waveform applied to the spacer (upper), flexion-extension angle waveform (lower)
The patterns and ranges of the axial force and of the flexion-extension conditions are reported in Figure 3: the internal-external rotation has been set to a fixed value because the knee spacer does not allow any rotation about the vertical axis while the lower sliding block has been left free to move in order to adapt its movement to the one imposed by the femoral condyles during flexion-extension. The maximum value of the imposed load was set to 1300 N, half of the load that normally acts during walk- ing, considering that the patient, during the rehabilitation period that follows the implant of the spacer, should walk with the aid of crutches, thus reducing the total amount of the load that acts on the knee. In order to asses the mechanical reliability of the device even in case the patient should not follow the prescription of using crutches, three more tests have been performed on the S-size device imposing a verti- cal load equal to 2600 N corresponding to the full bearing condition. In both the con- ditions, the test frequency was set to 0.5 Hz.
In order to mimic the in vivo environmental conditions of the tissues that sur- round the spacer, on the MTS knee simulator the device was located into a sealed chamber and kept constantly lubricated by means of a mixture of water and 25 % bovine serum at the temperature of 37°C. This mixture is recognized as having lubri- cating properties similar to those of sinovial fluid [3].
Fatigue Tests on the Tibial Tray
The test goal was determining the fatigue behavior of the tibial plate of the spacer,
basically following the prescription of ASTM F 1800 – 97 standard: this set-up sug-
gests to place the specimen on the testing machine in the configuration of a cantilever
beam by cementing it to the lower grip and fixing to its centerline through a clamp as
illustrated in Figure 4.
Fig. 4. Experimental set-up for the fatigue tests on the tibial tray
Table 1. Fatigue tests
parameters Minimum load [N] Maximum load [N] Frequency [Hz]
60 600 2.5
50 500 3
40 400 4
32.5 325 4.5
In order to determine which of the three sizes of the spacer could be the weakest, pre- viously to the fatigue tests, three static tests were performed on a plate of the S- size, on a plate of the M-size and on a plate of the L-size under displacement control at the speed of 2 mm/min until break was detected.
In the fatigue tests a sinusoidally variable load was applied with maximum and minimum values corresponding to those reported in Table 1; tests were performed until either break was detected or 500,000 cycles were run, accordingly to the expected time of use of the spacer (test frequency is reported in Table 1 as well).
Tests have been repeated on three samples of the tibial tray for each load value.
Results
Cyclic Tests on the Whole Spacer
As concerns the mechanical resistance of the whole spacer, this has been assessed by the fact that no failure was recorded for any spacer at the end of the loading sessions:
half of the normal load acting on the knee during walking has been applied on the
spacer, due to the fact that the patient is supposed to walk in the interim period with
the aid of crutches, and the frequency of testing was half the normal one, too, consid- ering that the same patient should walk slower than a normal person does. Even the eventuality of a patient that does not follow medical prescription of using crutches has been investigated by doubling the applied load with the good result of absence of failure in this case too.
Fatigue Tests on the Tibial Tray
As regards the tests on the reliability of the tibial plate of the spacer, results of the pre- liminary static tests are reported in Figure 5: the plate of S-size spacer, as expectable, has turned out to be the weakest and, for this reason, the fatigue tests have been per- formed on plates of that size. Results from fatigue tests (Wöhler curve) are reported in Figure 6: such a curve reports on the x-axis the number of cycles reached for each tested spacer and on the y-axis the maximum load at which the spacer has been tested. The plot reports also the 95 % and 99 % confidence bands, that represent, in case one hundred specimens should be tested, the interval containing 95 and 99 fail- ure points, respectively. The arrow rising from the three coinciding points at 325 N load level means that the tests have been stopped after 500,000 cycles, in accordance to the expected lifetime of the device.
Fig. 5. Result from static tests on the three sizes of the tibial tray
Fig. 6. Results of fatigue tests reported in the Wöhler curve
Discussion
At the time the infection arises, the surgeon must face three main problems, here reported in chronological sequence: i) the infection must be defeated as soon as pos- sible and without risk of recurrences together with an immediate relief of pain; ii) in the interim period before the new prosthesis is implanted, the patient should live as much as possible a “normal” life, in terms of possibility of movements and deambula- tion; iii) the reimplantation of the new prosthesis must be facilitated, trying to avoid any factor that may cause bone loss or difficulties in exposure at surgical time.
The three above described requirements stimulated the surgeons to move from the use of static spacers to mobile spacers, in particular after assessing the reliability of the latter as regards the efficacy of the infection treatment [5, 6].But the great advan- tages that may come from the use of a mobile spacer must be searched in the possibil- ity of movement for the patient in the interim period, resulting in an increase of the range of motion (ROM) of the knee after the second implant [5, 10, 13] , in a lower incidence on bone loss due to resorption [6] and in an easier operating act at the time of reimplantation [10].
Different mobile spacers have been proposed with different materials for the articu- lating components, either antibiotic addicted PMMA on PMMA or metal on Polyethyl- ene (PE): metal on PE implants do not have the ability of antibiotic release and may increase the risk of reinfection due to the presence of additional surface area for bacte- ria to adhere; on the other hand, the proposed PMMA on PMMA mobile spacers have been manufactured by means of intraoperatively moulds, that means a non-controlled manufacturing procedure with the risk of unknown mechanical resistance properties.
Some authors have implicitly asked the knowledge of these properties. For exam- ple, Fehring et al.[6], who found little differences in ROM between static and mobile spacers, lamented that the conservative formal physical therapy in the interim period may have influenced this poor result and suggested that a more aggressive rehabilita- tion should improve the mobility of the knee. On the other hand, the increase of such rehabilitation facility could be responsible for any mechanical failure of the implanted device and so the in vitro evaluation of the mechanical performances of the mobile spacer turns out to be a key issue to assess its suitability in a pre-clinical phase: the six month period of permanence inside the knee of the patient can easily be considered long enough to assign the spacer the name of “prosthesis” with the consequent need of a complete experimental procedure that takes into account the problems that can arise in the long run, particularly related to fatigue.
This study has been made possible thanks to the fact that the spacers were indus- trially manufactured, with a controlled manufacturing procedure, thus allowing to have specimens whose characteristics do no depend (differently from those proposed in literature) on the particular parameters under which they have been obtained:
quantity of antibiotic in the PMMA mass, temperature and humidity of the environ- ment in which they have been moulded (operating theatre), moulding technique. In particular, having a controlled percentage of antibiotics is an advantage both for the infection healing capacity of the spacer and for its mechanical resistance perfor- mances, that are strongly influenced by this factor.
In order to assess the mechanical reliablity in time of the device both cyclic tests on
the whole spacer and fatigue tests on the tibial tray have been performed and some
considerations can be drawn when evaluating the above reported results. In the cyclic tests on the whole spacer, the choice of using over-loading factors (very high load, great number of cycles, wide range of motion) are a commonly used procedure in the designing phase of an experimental set-up used to assess the mechanical reliability of an endoprosthesis. In fact, the biological response of the host tissue to the insertion of the device and the complexity of the anatomy and physiology of the prosthetic joint are two factors that are definitely very difficult to simulate in in vitro tests: our experi- mental set-up, for example, is able to take into account, as a biological factor, only the lubricating mixing composition and its temperature that are similar to those found in
vivo in the prosthetic knee. As a consequence, in order to give consistence to theresults coming from the tests, a sort of “safety factor” is introduced by applying mechanical conditions that are surely much more severe than the one the device will be in vivo subjected to (in our case we increased the magnitude of the mechanical input parameters such as load, number of cycles, flexion-extension ROM).
Also in the case of the fatigue tests on the tibial tray, the same consideration above expressed on the testing experimental conditions that are applied to the device can be valid and some factors must be taken into account when examining the results. The loading configuration proposed by ASTM F 1800 – 04 is defined in literature as an exaggeration of reality as it prescribes to load the tibial tray as if either in the medial or in the lateral side the bone support should completely lacks: this condition is far from reality where a resorption of the tibial bone as well as the formation of fibrous tissue between the bone and cement may cause inadequate support to the tray, but the complete absence of one tibial emi-plate has never been observed. Therefore, the load that is applied to the spacer cannot surely be the physiological one (about 2000 N), that would produce failure of the tibial plate due to the particular prescribed testing configuration: the choice of the right load to apply has been discussed in literature [1]
with the final suggestion of a 500 N vertical load repeated for five million cycles (cor- responding to five year waking of the patient) as the preferred loading condition to assess the fatigue reliability of the tibial tray of a total knee arthroplasty (TKA): con- sidering that the patient bearing a knee spacer has an activity level well below the one of a patient bearing TKA, the value of the vertical load of 325 N assessed for a life of 500,000 cycles could be high enough to ensure the fatigue reliability of the knee spacer tibial tray.
As a conclusion of this study, even if the tested spacer can be defined as a “tempo- rary” device, the authors’ consideration is that, in the pre-clinical phase, the experi- mental procedure that is run in order to assess its biomechanical reliability must be as much complete as possible and very close to the one that is usually applied to a
“permanent” device: in this light, the spacers that have been tested have shown a good behaviour, with a consequent high enough degree of suitability for the clinical use.
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