JBJS | Article

The Journal of Bone & Joint Surgery, Volume 80, Issue 1

Articles | January 01, 1998

Early Inducible Displacement of Tibial Components in Total Knee Prostheses Inserted with and without Cement. A Randomized Study with Roentgen Stereophotogrammetric Analysis*

SÖREN TOKSVIG-LARSEN, M.D., PH.D.†; LEIF RYD, M.D., PH.D.†; ANDERS LINDSTRAND, M.D., PH.D.†, LUND, SWEDEN

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Investigation performed at the Department of Orthopedics, University Hospital, Lund

The Journal of Bone & Joint Surgery. 1998; 80:83-9

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Abstract

The fixation of tibial components randomized to insertion with or without cement in twenty-six knees was examined for inducible displacement at six weeks and one year postoperatively with use of roentgen stereophotogrammetric analysis. Furthermore, migration was studied during the first two postoperative years. Inducible displacement was found in all knees at both the six-week and the one-year follow-up examination, but no differences were detected with respect to the type of fixation (p > 0.05). All tibial components migrated for as long as one year postoperatively, after which most stabilized. No difference was found between the groups with respect to migration during the first two years postoperatively (p > 0.05), with the exception of subsidence of the component, which was found to be 0.0 ± 0.1 millimeter (mean and standard error of the mean) for the components inserted with cement and 0.5 ± 0.1 millimeter for the components inserted without cement (p < 0.01). Migration after one year was the same for both groups. We found a relationship between inducible displacement at six weeks and at one year as well as one between inducible displacement and migration at one year. To our knowledge, the present study is the first in which the micromotion of an interference-fit prosthesis was found to be similar to that of a device inserted with cement. The results of the present study emphasize the importance of the initial prosthetic fixation.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

The fixation of prosthetic components remains a challenge, both clinically and scientifically. Histological studies have shown that a soft fibrous-tissue membrane usually forms between the bone and the prosthetic implant after a joint arthroplasty^22,38. When an implant has been inserted with cement, this membrane is seen on roentgenograms as a radiolucent line even in knees with an excellent clinical result^16,20. When an implant has been inserted without cement, and different kinds of surfaces have been used to provide an interference fit for immediate fixation and later osseous ingrowth, fibrous fixation also is predominant^3,6.

The development of the fibrous membrane has been attributed to a number of factors, such as operative trauma, inclusion of debris, and enzymatic activity^5,12. Mechanical factors resulting in relative motion between the implant and the bone have been implicated both after arthroplasties with cement^8,9,36 and after those without cement^4,25. Laboratory studies^36,37 as well as animal models³³ have shown that motion occurs at the implant-bone interface. In vivo roentgen stereophotogrammetric analysis of micromotion between the tibia and the tibial component has demonstrated inducible relative displacement in response to external forces one to two years after a successful arthroplasty^26,35. Micromotion of fifty to 150 micrometers at the implant-bone interface immediately after implantation may prevent ingrowth of appositional bone and, in particular, ingrowth of osteoid with subsequent formation of bone within the pores¹¹.

The purpose of the present study was to investigate the fixation of tibial components, which had identical inserts and stems², during the initial two years after implantation with and without cement.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were the Alfred Österlunds Stiftelse, Lunds Sjukvårdsdistikt, Lund University Medical Faculty, Greeta and Johan Kocks Stiftelse, Malmöhus Läns Landsting, Socialstyrelsens fonder, Tore Nilsson Fond, and Stiftelsen för bistånd åt vanföra i Skåne, Nutek, koneung Gustav V's 80-årsfond, The Craaford Foundation, and the Medical Research Council project 09509.

†Department of Orthopedics, University Hospital, S-221 85 Lund, Sweden. E-mail address: soren.toksvig-larsen@ort.lu.se (Dr. Toksvig-Larsen).

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Fig. 1 Drawing showing the general locations of the tantalum-ball markers (circles) in the left tibia and in the tibial polyethylene bearing insert.

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Fig. 2 Graph showing the inducible displacement of the components (mean and standard error of the mean) for each group at six weeks and at one year, according to the positions studied in the stress examinations.

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Fig. 3 Graph showing the migration of the components (mean and standard error of the mean) during the first two years.

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TABLE I DATA ON THE KNEES*

*The values are given as the mean with the range in parentheses.

	With Cement	Without Cement
The Hospital for Special Surgery knee score¹⁷(points)
Preoperative	59 (46—79)	59 (42—87)
One year	92 (84—96)	88 (75—99)
Two years	93 (90—97)	88 (74—98)
Walking distance (m)
Preoperative	305 (50—700)	445 (50—2000)
Two years	2227 (1000—3000)	2736 (50—6000)
Extension (degrees)
Preoperative	6 (0—15)	6 (-10—30)
Two years	0.5 (0—5)	1 (0—15)
Flexion (degrees)
Preoperative	115 (90—130)	120 (90—130)
Two years	115 (100—130)	110 (85—130)

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TABLE I DATA ON THE KNEES*

*The values are given as the mean with the range in parentheses.

	With Cement	Without Cement
The Hospital for Special Surgery knee score¹⁷(points)
Preoperative	59 (46—79)	59 (42—87)
One year	92 (84—96)	88 (75—99)
Two years	93 (90—97)	88 (74—98)
Walking distance (m)
Preoperative	305 (50—700)	445 (50—2000)
Two years	2227 (1000—3000)	2736 (50—6000)
Extension (degrees)
Preoperative	6 (0—15)	6 (-10—30)
Two years	0.5 (0—5)	1 (0—15)
Flexion (degrees)
Preoperative	115 (90—130)	120 (90—130)
Two years	115 (100—130)	110 (85—130)

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Twenty-six patients who had osteoarthrosis of the knee unilaterally and one patient who had it bilaterally had a knee arthroplasty with use of the porous-coated anatomic total knee prosthesis (PCA Universal; Howmedica, Rutherford, New Jersey) with a porous-coated anatomic modular cruciform stem tibial component. One patient was excluded because of a cardiac infarction that had occurred immediately before the first roentgen stereophotogrammetric stress analysis, and one was excluded because of technical problems with the roentgen stereophotogrammetric analysis. This left twenty-six knees (twenty-five patients) for the final analysis. The study was approved by the Ethics Committee of the Medical Faculty of Lund University. Informed consent was obtained from all patients.

During the operation, the patients were randomized to either insertion of the tibial component with cement (eleven knees) or insertion of the component without cement (fifteen knees). The femoral component was inserted without cement, and the patella was not resurfaced. There were ten men and fifteen women (one woman had the procedure bilaterally) who had a mean age of seventy-one years (range, fifty-nine to eighty-two years). The mean body weight was seventy-nine kilograms (range, forty-six to 100 kilograms). The mean preoperative hip-knee-ankle angle was 12 degrees of varus alignment (range, 22 degrees of varus to 3 degrees of valgus alignment). At the two-year follow-up examination, twenty-two patients (twenty-three knees) remained in the study, as two patients had died and one roentgen stereophotogrammetric study could not be analyzed.

All bone cuts were done with an internally cooled saw-blade, and room-temperature saline solution was used as a coolant³⁴. The position of all prostheses was satisfactory. The cementing technique included vacuum-mixing of the cement, lavage, and pressurization²¹. The cement was applied to the horizontal cut surface but not around the stem. The patients who had insertion of the tibial component with cement were allowed full weight-bearing postoperatively. The patients who had fixation of the tibial component without cement were instructed to protect the involved limb by partial weight-bearing with use of crutches during the first six weeks. Only ten of the fifteen patients in that group complied with these instructions totally or partially during the first six-week period.

All preparations for roentgen stereophotogrammetric analysis³² were made during the operation. Tantalum-ball markers were inserted into the proximal part of the tibia and into the tibial polyethylene insert to obtain distinct points of identification on roentgenograms (Fig. 1).

Roentgen Stereophotogrammetric Analysis

We defined migration as displacement of the implant with respect to the bone over time and in the absence of external forces. Subsidence was defined as distal migration of the implant along the tibial axis (-Y) (Fig. 1). We defined inducible displacement as reversible motion of the prosthesis relative to the bone induced by external force. Reference roentgen stereophotogrammetric analysis was performed immediately after implantation and at six weeks, six months, one year, and two years postoperatively. Roentgen stereophotogrammetric stress analysis was performed six weeks and one year postoperatively to study inducible displacement.

Six weeks after the operation, when the patients were walking satisfactorily, roentgen stereophotogrammetric stress analysis was performed with the knee in five different positions to study inducible displacement. Testing was performed with the patient supine (position one), standing on the lower limb that had been operated on (position two), standing on the lower limb that had been operated on with that limb externally rotated and then internally rotated with a torque of ten newton-meters (positions three and four), and squatting to approximately 60 degrees of flexion of the knee with use of the arms for support (position five). The knee could not be flexed more than 60 degrees because, when it was, the x-ray beam for the anteroposterior roentgenogram was obscured by the buttocks of the patient. An identical stress analysis was performed one year postoperatively. During each stress examination, motion was investigated between positions one and two, which represented the difference between non-weight-bearing and weight-bearing; between positions three and four, which represented the situation in which the largest inducible displacements previously have been found²⁶; and between positions two and five, which represented the change in weight-bearing during flexion of the knee (for example, during stair-climbing or rising from a chair).

The results were given either as segment motion—that is, the translation of the geometric center or rotation of the prosthesis—or as maximum total point-motion—that is, the translation of the single prosthetic marker that moved the most (Fig. 2). The overall maximum total point-motion—that is, the total three-dimensional vector translation of the marker that moved the most in any one position during the entire series of stress examinations—was given as a simple way to denote the magnitude of the inducible displacement. The accuracy of the roentgen stereophotogrammetric system in this application is to within 0.3 degree for single-axis rotation and to within 0.2 millimeter for translation²⁶. The prostheses were classified as stable or as continuously migrating, according to the magnitude of the migration noted between the one and two-year follow-up examinations³⁰. Continuous migration was defined as maximum total point-motion of more than 0.2 millimeter between the two examinations.

Roentgenograms of the entire lower limb in the weight-bearing position were made preoperatively and postoperatively with use of a calibrated frame in order to determine the mechanical axis—that is, the hip-knee-ankle angle²³. An angle of less than 180 degrees was considered to represent varus alignment.

The knees were rated, according to the system of The Hospital for Special Surgery¹⁷, preoperatively and at the one and two-year follow-up examinations.

Statistical analysis was performed with use of the analysis of variance test, the t test, and regression analysis; a probability level of p < 0.05 indicated significance.

Results

Introduction | Materials and Methods | Results | Discussion | References

All patients were satisfied with the result, and the knee scores, according to the system of The Hospital for Special Surgery¹⁷, were rated as good or excellent (Table I). The mean knee score at the latest follow-up examination was 90 points (range, 74 to 98 points). With the numbers available, no significant difference could be detected between the mean score of 93 points for the knees in which the tibial component had been inserted with cement and the mean score of 88 points for those in which the implant had been inserted without cement (p = 0.07; power, 0.79). Also, the findings for the two groups were similar with respect to the distance that the patients could walk postoperatively (p = 0.46) and the range of motion (p = 0.66 for extension and p = 0.42 for flexion) (Table I).

Inducible Displacement

Inducible displacement was found in all knees at both the six-week and the one-year follow-up evaluation (Fig. 2). At six weeks, the maximum total point-motion between positions one and two was 0.5 ± 0.1 millimeter (mean and standard error of the mean) in the group that had fixation with cement and 0.6 ± 0.1 millimeter in the group that had fixation without cement (p = 0.49). In the test of rotation (the difference between positions three and four), the maximum total point-motion was 0.5 ± 0.1 millimeter for both groups (p = 0.81). During squatting (the difference between positions two and five), the maximum total point-motion was 0.5 ± 0.1 millimeter for both groups (p = 0.98). With the numbers available, no significant differences were found in medial-lateral, superior-inferior, or anterior-posterior translation or in rotation about the corresponding axes.

At the one-year follow-up evaluation, the maximum total point-motion between positions one and two was 0.3 ± 0.1 millimeter for the group that had fixation with cement and 0.5 ± 0.1 millimeter for the group that had fixation without cement (p = 0.053). Between positions three and four, the maximum total point-motion was 0.7 ± 0.1 millimeter for the group that had fixation with cement and 0.5 ± 0.1 millimeter for the group that had fixation without cement (p = 0.26). Between positions two and five, the maximum total point-motion was 0.5 ± 0.1 millimeter in both groups (p = 0.68). With respect to the six cardinal axes, a significant difference was found only in the y-translation (subsidence), which was -0.11 ± 0.03 millimeter for the group that had fixation with cement and 0.05 ± 0.04 millimeter for the group that had fixation without cement (p = 0.008). No significant difference was found in the maximum total point-motion in the different positions between the six-week and the one-year stress examination (p = 0.31 for motion between positions one and two, p = 0.68 for motion between positions three and four, and p = 0.79 for motion between positions two and five; paired t test). Indeed, a significant correlation was found between the maximum total point-motion values at the two follow-up examinations (r² = 0.20, p = 0.035; simple regression analysis), which represent the maximum cyclic instability.

Migration

The maximum total point-motion was 0.7 ± 0.3 millimeter (mean and standard error of the mean) and 0.9 ± 0.1 millimeter at the six-week follow-up examination, 1.0 ± 0.2 millimeter and 1.4 ± 0.2 millimeters at the six-month evaluation, 1.0 ± 0.2 millimeter and 1.4 ± 0.2 millimeters at the one-year evaluation, and 1.0 ± 0.1 millimeter and 1.5 ± 0.2 millimeters at the two-year examination for the knees that had fixation with cement and those that had fixation without cement, respectively (p = 0.061, repeated-measures analysis of variance, six weeks, six months, one year, and two years; power, 0.99) (Fig. 3).

The proportion of continuously migrating prostheses was the same in the two groups. At the two-year follow-up examination, six prostheses were stable and three had migrated continuously in the group that had fixation with cement compared with ten that were stable and four that had migrated continuously in the group that had fixation without cement. Only subsidence differed between the two groups; the prostheses that had been inserted with cement subsided 0.0 ± 0.1 millimeter and the prostheses that had been inserted without cement subsided 0.5 ± 0.1 millimeter by the two-year follow-up evaluation (p = 0.008).

A significant but weak correlation was found between migration during the first year and displacement measured during the stress examinations at the one-year follow-up examination (r² = 0.20, p = 0.04; simple regression analysis, maximum total point-motion for migration and maximum total point-motion for inducible displacement).

A weak correlation was detected between the preoperative, but not the postoperative, deformity of the knee (the hip-knee-ankle angle) and the inducible displacement at the six-week follow-up examination (r² = 0.17, p = 0.044) and the migration during the two-year period of follow-up (r² = 0.28, p = 0.013).

No correlation was found between The Hospital for Special Surgery knee score and the inducible displacement at the six-week or the one-year follow-up evaluation or between the migration during the first two postoperative years and the clinical outcome for both groups combined. However, in the group that had fixation with cement, an inverse correlation was detected between The Hospital for Special Surgery knee score and the inducible displacement at one year (r² = 0.87 and p = 0.0007 for the maximum total point-motion between positions one and two; r² = 0.54 and p = 0.04 for the maximum total point-motion).

No differences were found with respect to inducible displacement between the stable prostheses and those showing continuous migration. Patients who were less than seventy years old had a greater tendency for continuous migration of the prosthesis (p = 0.07). None of the remaining demographic, clinical, or radiographic parameters were associated with the results of the roentgen stereophotogrammetric analysis.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Cyclic inducible displacement has been shown to be a result of the formation and maintenance of a fibrous membrane between the bone and the implant^22,38. The interface may be regarded as a connection between an implant and the surrounding bone, with or without an intervening soft-tissue membrane. Inducible displacement theoretically can take place within the implant itself (that is, between the loose-fitting parts of the metal-backed tibial component), as a slip between the implant and the membrane, as a deformation within the membrane, or as an elastic deformation within the bone²².

As far as we know, we are reporting the first randomized in vivo study comparing the inducible displacement of tibial components inserted with cement with that of components of identical design (porous-coated anatomic cruciform tibial components [PCA Universal; Howmedica]) inserted without cement. No difference was found, an observation that is in contrast to the results reported in a previous study comparing devices fixed with and without cement²⁷. The difference between the findings of the two studies was due to the decrease in the cyclic micromotion of the components inserted without cement in the present study.

An important finding in the present study was that the magnitude of the inducible displacement appeared to be established by six weeks; there was no change in the compliance of the interface, of either the prostheses inserted with cement or those inserted without cement, with a longer duration of follow-up. The rationale behind the investigation of inducible displacement at six weeks was that a fibrous-tissue membrane would not be established at that time. As no increase in inducible displacement was detected at the one-year follow-up evaluation, we concluded that inducible displacement is partly due to osseous compliance—that is, motion of an elastic nature within the bone itself. This compliance is totally recoverable, as shown in a previous study²⁶. This conclusion is supported by the findings of an analytical study that demonstrated considerable compliance in response to load within an intact tibia²⁴.

Inducible displacement of fifty to 150 micrometers at the implant-bone interface immediately after implantation may prevent appositional bone ingrowth and, in particular, ingrowth of osteoid with subsequent formation of bone within the pores¹¹. However, inducible displacement may occur as elastic micromotion within the cancellous bone and not as a slip in the interface; if that is the case, such micromotion may not preclude osseous ingrowth.

We did not find any difference in the magnitude of inducible displacement between the patients who had fixation of the prosthesis with cement and were allowed to bear weight and those who had fixation without cement and were instructed to perform partial weight-bearing. Protected weight-bearing postoperatively is controversial because all patients do not comply. The findings of our studies suggest that the issue of weight-bearing or non-weight-bearing is irrelevant. However, physiotherapy and range-of-motion exercises place a substantial load on the interface.

Induced displacement may cause fluid to move in and out of the interface—that is, the so-called effective joint space³¹—and may cause fatigue with the accumulating load cycles. Irrespective of the predominant failure scenario, a relationship between migration and inducible displacement would be expected. This was demonstrated at the one-year follow-up examination in the present study as well as in a previous study¹³.

We found that the magnitude of inducible displacement in both the initial and the mature stage was the same regardless of the type of fixation. This finding may indicate that both the prosthetic design and the operative technique are factors in the achievement of optimum initial fixation. Because no significant difference was found with respect to continuous migration, we concluded that the two groups had a similar prognosis (p > 0.05)³⁰. In a non-randomized study, Ryd et al.²⁹ showed that the addition of cement to the porous-coated anatomic primary prosthesis reduced the migration at one year from a mean maximum total point-motion of 1.9 to 0.8 millimeter. Albrektsson et al.¹ found that use of cement reduced the migration at one year from a mean maximum total point-motion of 1.5 to 0.5 millimeter in a randomized study of knee arthroplasty with the Freeman-Samuelson prosthesis. Cook et al.⁷ found no osseous growth in a first-generation implant inserted without cement, whereas second-generation implants showed growth into 2 to 40 per cent of the interface⁶. The same second-generation implant, with screw fixation, was shown to yield little inducible displacement on roentgen stereophotogrammetric analysis²⁸. The findings of these studies^1,28-30 indicate that fixation of tibial components without cement has improved and currently is equal to fixation of such components with cement. In the present study, the correlation between the preoperative alignment of the knee and inducible displacement was interesting and unexpected. A possible explanation is the fact that the tibial bone cuts were made more distally, in the less dense and hence lower-modulus cancellous part of the tibia¹⁵ to correct the preoperative malalignment. The more severely the knee is malaligned before the operation, the greater the differences in bone stiffness that should be expected between the medial and lateral condyles. This correlation was verified in an experimental study by Volz et al.³⁶.

In conclusion, we showed that when there is little inducible displacement of a prosthesis after six weeks there will be little inducible displacement after one year and little migration after two years. A number of recent reports have shown that both knee and hip prostheses that migrate continuously are the only ones that eventually will loosen^10,18,19,27. Such continuous migration was linked to inducible displacement in a previous study¹⁴. These findings demonstrate that loosening is established in the early period; the most decisive moment for the survival of the prosthesis is probably during the operation itself. At that time, a number of unrelated factors, such as the patient, the surgeon, the prosthesis, and the technique, are brought together in a unique combination. Although our conclusion pertains only to our patients, the validity of these suggestions may be worth considering in a general sense.

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Introduction | Materials and Methods | Results | Discussion | References

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Fig. 1 Drawing showing the general locations of the tantalum-ball markers (circles) in the left tibia and in the tibial polyethylene bearing insert.

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Fig. 3 Graph showing the migration of the components (mean and standard error of the mean) during the first two years.

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TABLE I DATA ON THE KNEES*

*The values are given as the mean with the range in parentheses.

	With Cement	Without Cement
The Hospital for Special Surgery knee score¹⁷(points)
Preoperative	59 (46—79)	59 (42—87)
One year	92 (84—96)	88 (75—99)
Two years	93 (90—97)	88 (74—98)
Walking distance (m)
Preoperative	305 (50—700)	445 (50—2000)
Two years	2227 (1000—3000)	2736 (50—6000)
Extension (degrees)
Preoperative	6 (0—15)	6 (-10—30)
Two years	0.5 (0—5)	1 (0—15)
Flexion (degrees)
Preoperative	115 (90—130)	120 (90—130)
Two years	115 (100—130)	110 (85—130)

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TABLE I DATA ON THE KNEES*

*The values are given as the mean with the range in parentheses.

	With Cement	Without Cement
The Hospital for Special Surgery knee score¹⁷(points)
Preoperative	59 (46—79)	59 (42—87)
One year	92 (84—96)	88 (75—99)
Two years	93 (90—97)	88 (74—98)
Walking distance (m)
Preoperative	305 (50—700)	445 (50—2000)
Two years	2227 (1000—3000)	2736 (50—6000)
Extension (degrees)
Preoperative	6 (0—15)	6 (-10—30)
Two years	0.5 (0—5)	1 (0—15)
Flexion (degrees)
Preoperative	115 (90—130)	120 (90—130)
Two years	115 (100—130)	110 (85—130)