The souter-strathclyde elbow prosthesis in rheumatoid patients : an in-depth clinical, radiological and biomechanical analysis

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biomechanical analysis

Lugt, J.C.T. van der

Citation

Lugt, J. C. T. van der. (2010, March 11). The souter-strathclyde elbow prosthesis in rheumatoid patients : an in-depth clinical, radiological and biomechanical analysis. Retrieved from https://hdl.handle.net/1887/15067

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7

MIGRATION OF THE HUMERAL COMPONENT OF THE SOUTER-STRATHCLYDE ELBOW PROSTHESIS

J.C.T. van der Lugt E.R. Valstar S.W. Witvoet-Braam R.G.H.H. Nelissen

Accepted J Bone Joint Surg Br. 2009

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Abstract

Mechanical loosening, which starts with early onset prosthetic migration, is the major reason for revision of the Souter Strathclyde elbow prosthesis. In a prospective observa- tional study on eighteen Souter Strathclyde elbow prostheses we evaluated migration patterns with radiostereometric analysis (RSA). In 2002 we presented the short-term results after two years of follow-up. Migration was assessed along the x-, y, and z –axis, overall micromotion was expressed as Maximum Total Point Motion (MTPM). Prosthetic alignment and radiolucent lines (RLLs) were examined on conventional radiographs. All humeral components showed increased and irregular migration patterns after a mean follow-up of 8.2 years. Four humeral components were revised at the long-term. Two year MTPM in the revised prostheses was 1.8 mm (SD 1.0), in the non revised 0.7 mm (SD 0.5, p=0.01). The majority of humeral components migrated into external rotation, resulting in an anterior and varus tilt. The ulnar components remained stable.

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Introduction

The most common reason for failure of the Souter-Strathclyde total elbow prosthesis is mechanical loosening of the humeral component.^1,2,3 In a recent study of 204 Souter- Strathclyde total elbow prostheses, we found a survival rate of 77 % after 10 years with endpoint revision.⁴ However, if radiological loosening – complete and progressive radio- lucency t 1 mm – of the prosthesis was taken as an endpoint, the survival rate decreased to 65 % after 10 years.⁵

The explanation for the high mechanical loosening rate, especially for the humeral component, is still not clarified yet. Some authors relate the mechanical failure rate to the poor bone stock in rheumatoid arthritis, others found that the elbow axis is shifted postoperatively. Since the latter will result in increased moment arms at the prosthesis-bone interface, loosening of the humeral component will be induced during daily activities.^6,7 Contradictory to this theory, little influence was found between prosthesis position and survival.⁵ This might be due to the inaccuracy of prosthesis alignment measurement on radiographs as well as to the endpoint definition for prosthesis survival, being revision of the prosthesis. In order to evaluate the mechanical behaviour of the elbow prosthesis, migration patterns can be studied. Radiostereometric Analysis (RSA) is known for its accuracy of prosthesis migration measurements. In order to elucidate the mechanism of the mechanical loosening process of the Souter-Strathclyde elbow prosthesis an RSA study was started in 1995. In 2002 we presented the short-term results after two years of follow-up.⁸

Patients and methods

Twenty one successive elbow prostheses were implanted between 1995 and 1997 in eighteen patients diagnosed with rheumatoid arthritis. In three patients a bilateral procedure was performed. Three elbows were excluded, two because of instable markers and one patient died within three months of follow-up.

We used the Souter-Strathclyde total elbows prosthesis (Stryker UK Ltd, Newbury, UK), a semiconstrained type of prosthesis The indication for operation was pain in presence of severe joint destruction (Larsen grade 4-5).⁹ The mean age at operation, after exclusion the three elbows, was 62 years (range 50 to 73 years). The humeral components were sized medium in ﬁfteen elbows, small in two and large in one elbow. No long-stemmed humeral components were used in this series. The ulnar components were sized medium in thirteen elbows, small in three and metal backed with snapﬁt in one elbow. The mean follow-up was 8.2 years (SD 3.6, see Table 1). The protocol was approved by the medical ethics committee and all patients gave informed consent.

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The RSA-humeral component had three 2-mm Vitallium markers attached to the metal of the component, whereas in the RSA-ulnar component four 0.8-mm tantalum markers were inserted into the polyethylene of the component (Figure 1).⁹ We inserted three to eight 1-mm tantalum markers into the bone with a custom made device. These markers represented a local coordinate system in relation to which the motion of the components was calculated. The locations of the box markers deﬁned the global coordinate system and were used to assess the roentgen foci positions. These positions were accurately measured with a mechanical measuring device that had an accuracy of Table 1. Characteristics of the 18 elbow prostheses. The eight elbows ‘at risk’ for loosen-

ing according Valstar et al. (2002) are given.

Elbow number

Age Sex Side Size humeral component

Size ulnar component

At risk at two years

Revision FU

1 73 M L Medium Small No No 23

2 70 F R Medium Small No Yes^# 67

3 54 F R Small Small No No 135

4 59 F L Medium Medium No No 114

5 71 F R Medium Medium* No Yes 121

6 60 F R Small Small No Yes 96

7 52 F R Medium Medium No No 134

9 50 F L Medium Medium Yes No 132

10 61 M R Medium Medium No No 132

12 57 F R Medium Medium Yes No 24

17 57 M L Large Medium Yes No 86

Mean (SD) 62 (7.2) 8.2 (3.6)

FU = Follow-up in months (revision is endpoint)

* Metal backed ulnar component with snapﬁt option because of peroperative instability during trial reduction.

# Revision of the ulnar component into a metal backed component with snapﬁt option 14 months after the primary procedure because of recurrent dislocations. This elbow prosthesis was followed for 67 months in total and no other complications were noticed.

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0.001 mm. Positive directions for translations along the coordinate axes were lateral to medial (transverse axis), distal to proximal (longitudinal axis), and posterior to anterior (sagittal axis). Positive directions for rotations about the coordinate axes were anterior tilt (transverse axis / x-axis), internal rotation (longitudinal axis / y-axis), and varisation (sagittal axis / z-axis). The overall 3-D migration, the Maximum Total Point Motion, of the prosthesis was measured at 1 and 2 year follow-up to correlate component position to overall prosthesis migration.

The radiographs were scanned with a Vidar VXR-12 scanner (Vidar, Lund, Sweden) at 150 dots per inch resolution and 8-bit gray scale resolution. The measurement of marker coordinates in the digitized radiographs, the three-dimensional reconstruction of the marker positions, and the micromotion analysis were done with MB-RSA (MEDIS specials, Leiden, The Netherlands. When bone markers moved more than 0.3 mm in relation to the other bone markers they are considered unstable and excluded from analysis.

The upper limits of the 95% conﬁdence interval for the accuracy of RSA measurements in three axes have already been calculated by Valstar et al.⁸ These accuracies were based on repeated scanning during the ﬁrst RSA-measurements postoperatively. RSA-measurements were performed post-operatively, at 3 months, 6 months, 12 months and at yearly intervals thereafter.

Prosthetic position and radiolucent lines (RLLs) were examined on conventional radiographs by using the Wrightington method (Figure 2)¹ Conventional radiographs were

A B

Fig. 1. The RSA-Souter-Strathclyde total elbow prosthesis in anteroposterior (A) and lateral view (B).

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made postoperatively and every two years thereafter in addition to the RSA radiographs.

Loosening was deﬁned as complete and progressive radiolucent lines of 1 mm and more around the component or migration of the component during the follow-up.

Revision of the elbow prosthesis was performed in presence of clinical symptoms and presence of prosthesis tilting with a potential for fracture of the humerus on conventional radiographs.

Statistics

A Linear Mixed Model was used to investigate the effect of prosthetic alignment on translations, rotations, and Maximum Total Point Motion (MTPM) with the level of signiﬁcance set at p<0.05. The MTPM, which is the total three-dimensional vector translation of the marker that moved most in any one position during the entire series, was used as a means of denoting the overall magnitude of the migration along the three orthogonal axes.¹⁰ A Student T-test was used for comparing the mean MTPM at one at two years for elbows that were still in situ and for elbows that were revised due to aseptic loosening (signiﬁcance set at p<0.05). All data were analyzed with SPSS version 14.0 (SPSS Inc., Chicago, IL, USA).

Results

As was reported earlier, clinical results improved comparing the preoperative with postoperative results. ^4,8

The humeral components were positioned within a range of 3 to 20 degrees varus alignment (mean varus 7 degrees SD 4.6) on the anterior-posterior view and within a Fig. 2. The Wrightington method for standardized measurements on conventional radiographs

(Trail et al.1999).

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range from 6 degrees extension to 8 degrees ﬂexion (mean ﬂexion 4 degrees SD 3.7) on the lateral view (Table 2).

The translation and rotation patterns of the humeral components were very irregular (Figure 3-5). Nine humeral components showed rotations more than 1.0 degree in the x and y- axis (range -2.4 to 2.3 and -3.9 to 1.7 respectively) and 3 humeral components had rotations more than 1.0 degree in the z-axis (range -1.4 to 2.5) during the ﬁrst two years postoperatively. Translations of the humeral component more than 1.0 mm were Table 2. Position in coronal and sagittal plane of the elbow prosthesis and radiolucent lines (RLLs)

of the humeral component according the Wrightington method.

Elbow number

Hap varus/

valgus (degrees)

Hlat ﬂexion/

extension (degrees)

Uap varus/

valgus (degrees)

Ulat ﬂexion/

extension (degrees)

Radiolucent lines (RLLs) at latest follow-up

1 6 2 0 -4 No

2 4 6 0 -6 No

3 5 8 0 -8 Uap 4 >1mm

4 4 4 2 -4 No

5 20 2 -5 -2 Complete humeral RLLs t1mm at 3 years

6 3 8 -3 -4 Complete humeral RLLs t1mm at 3 years

7 8 3 0 -6 No

8 3 8 0 -6 Complete humeral RLLs t1mm at 4 years

9 6 3 0 0 No

10 15 7 -3 -7 Hap 1,2 >1mm, Hap 3 >2mm, Hlat 1 >1mm

11 2 -6 0 -7 No

12 4 4 0 -2 Uap 4,5 >1mm

13 8 8 -2 -7 No

14 6 4 0 -4 No

15 5 6 0 -4 Hap 3 >1mm

16 5 -2 0 0 No

17 3 2 0 -7 No

18 11 5 -6 -8 Complete humeral RLLs t1mm at 3 years Mean (SD) 6.6 (4.6) 4.0 (3.7) -0.9 (2.0) -4.7 (2.5)

SD = Standard Deviation.

Varus = positive value.

Flexion = positive value

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Humeral translation z-axis

-1 0 1 2 3

0 3 6 12 24 36 48 60 72 84 96 108 120 132

Duration follow-up (months)

Translation (mm)

Fig 3. Sagittal translation (z-axis) of the humeral components during the follow-up. The 95 % conﬁdence interval of the accuracy of the RSA measurements is given (bold dark lines). A positive value indicates anterior tilt. The thick, coloured lines are the elbows which loosened and were revised. The dotted lines are the elbows which were deﬁned as ‘at risk’

according the study of Valstar et al.⁹

Fig. 4. Transverse rotation (x-axis) of the humeral components during the follow-up. The 95 % conﬁdence interval of the accuracy of the RSA measurements is given (bold dark lines). A positive value indicates anterior tilt. The thick, coloured lines are the elbows which loosened and were revised. The dotted lines are the elbows which were deﬁned as ‘at risk’

according the study of Valstar et al.⁹

Humeral rotation x-axis

-3 -2 -1 0 1 2 3 4 5

0 3 6 12 24 36 48 60 72 84 96 108 120 132 Duration follow-up (months)

Rotation ('anterior tilting', degrees)

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seen in two elbows for the x-axis (range -0.7 to 2.6) and in one elbow for the y and z-axis (range -0.5 to 1.1 and -0.4 to 1.3 respectively) during the ﬁrst two years of follow-up.

The four humeral components that were revised at a mean of 8 year follow-up had a mean MTPM of 1.4 mm (SD 0.8) whereas the non-revised humeral components had a mean MTPM of 0.9 mm (SD 0.9, p=0.34) at 12 months follow-up. At 24 months these values were 1.8 mm (SD 1.0) and 0.7 mm respectively (SD 0.5, p=0.01). Clinically these patients did not have any complaints and no radiological loosening was observed. No differences in MTPM between revised and non-revised elbows were seen after two years of follow-up.

Using a mixed model technique, no significant relation was found between varus/valgus and flexion/extension humeral alignment and translations along either of the three axes nor the MTPM during the first 2 years postoperatively. If we take the long-term RSA follow-up into account, the only significant effect found was an increase of MTPM of 0.18mm for every additional degree of varus position of the humeral component at the first postoperative radiograph (p = 0.02).

For the ulnar components, we did not observe any of these large micromotions or irregular patterns, although the precision of the measurements was lower than for the

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Fig. 5. Longitudinal rotation (y-axis) of the humeral components during the follow-up. The 95 % conﬁdence interval of the accuracy of the RSA measurements is given (bold dark lines). A positive value indicates internal rotation. The thick, coloured lines are the elbows which loosened and were revised. The dotted lines are the elbows which were deﬁned as ‘at risk’ according the study of Valstar et al.⁹

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Table 3. Mean migration of the humeral and ulnar components during the follow-up.

FU

Migration 6 12 24 36 48 60 72 84 96 108 120 132

Mean humeral translation in mm, (SD)

Number of elbows 18 18 14 3 8 13 11 11 12 8 8 5

X-axis

95 % CI (-0.13 – 0.13)

0.13 (0.53)

0.16 (0.72)

0.08 (0.75)

0.93 (1.68)

0.34 (1.29)

0.17 (1.05)

0.37 (1.10)

0.28 (1.21)

0.15 (0.99)

0.26 (0.99)

-0,18 (0.34)

-0.13 (0.35) Y-axis

95 % CI (-0.14 – 0.14)

0.15 (0.24)

0.25 (0.31)

0.35 (0.33)

0.47 (0.22)

0.42 (0.61)

0.60 (0.86)

0.57 (1.37)

0.79 (1.72)

1.00 (1.69)

1.56 (2.48)

0.76 (1.46)

1.10 (2.00) Z-axis

95 % CI (-0.34 – 0.34)

0.23 (0.45)

0.24 (0.39)

0.32 (0.32)

0.84 (0.72)

0.52 (0.55)

0.84 (1.04)

1.01 (1.83)

1.34 (2.04)

1.90 (2.62)

2.70 (3.37)

0.85 (1.30)

1.09 (1.71) Mean ulnar translation (mm,

SD)

Number of elbows 13 12 11 2 7 9 6 6 7 6 6 4

X-axis

95 % CI(-0.15 – 0.15)

-0.06 (0.26)

-0.13 (0.31)

-0.15 (0.38)

0.14 (0.20)

-0.30 (0.55)

-0.25 (0.56)

-0.25 (0.43)

0.17 (0.55)

-0.24 (0.44)

-0.24 (0.53)

-0.35 (0.51)

-0.60 (1.12) Y-axis

95 % CI (-0.05 – 0.05)

-0.05 (0.18)

-0.06 (0.27)

0.03 (0.17)

-0.14 (0.03)

-0.09 (0.30)

-0.12 (0.27)

-0.12 (0.26)

-0.05 (0.24)

-0.09 (0.27)

-0.14 (0.43)

-0.07 (0.48)

-0.00 (0.57) Z-axis

95 % CI (-0.17 – 0.17)

0.05 (0.13)

0.05 (0.12)

-0.01 (0.18)

0.06 (0.01)

0.07 (0.14)

0.05 (0.11)

-0.10 (0.24)

0.01 (0.15)

0.05 (0.14)

0.11 (0.26)

0.12 (0.23)

0.26 (0.43) Mean humeral rotation

(degrees, SD)

Number of elbows 14 14 11 2 6 9 8 8 8 4 7 4

X-axis

95 % CI (-0.56 – 0.56)

0.07 (1.11)

0.26 (1.28)

0.57 (0.70)

-0.22 (0.24)

1.40 (1.42)

1.01 (1.37)

1.74 (4.47)

1.09 (1.15)

4.08 (7.69)

2.40 (1.91)

0.93 (1.63)

1.67 (0.98) Y-axis

95 % CI (-0.43 – 0.43)

-0.48 (1.39)

-0.59 (1.24)

-0.44 (0.99)

-1.55 (1.50)

-0.84 (0.77)

-1.03 (1.65)

-0.21 (2.09)

-0.55 (1.30)

-1.08 (1.66)

-1.08 (1.29)

0.15 (1.64)

-0.27 (2.10) Z-axis

95 % CI (-0.23 – 0.23)

0.04 (0.63)

0.14 (0.84)

0.08 (0.50

-0.04 (0.31)

0.01 (0.53)

-0.09 (0.56)

0.48 (1.91)

-0.24 (0.48)

0.32 (2.24)

-0.41 (1.24)

0.19 (1.20)

-0.19 (1.36) Mean ulnar rotation (degrees,

SD)

Number of elbows 7 7 6 1 4 5 4 4 5 4 4 2

X-axis

95 % CI (-0.68 – 0.68)

0.19 (0.24)

0.34 (0.47)

0.23 (0.32)

0.24 (*)

0.29 (0.36)

0.17 (0.24)

0.05 (0.34)

0.10 (0.30)

0.04 (0.27)

0.39 (0.38)

0.44 (0.95)

0.53 (1.36) Y-axis

95 % CI (-0.34 – 0.34)

0.71 (1.82)

0.70 (2.11)

1.18 (2.81)

0.18 (*)

1.00 (3.16)

1.23 (2.54)

1.47 (3.02)

1.25 (3.64)

0.98 (2.82)

1.93 (3.37)

2.20 (2.99)

-0.86 (0.15) Z-axis

95 % CI (-0.16 – 0.16)

-0.16 (1.27)

-0.31 (1.41)

-0.57 (1.94)

-0.12 (*)

-0.60 (2.00)

-0.49 (1.78)

-0.62 (2.06)

-0.58 (2.35)

-0.36 (1.89)

-0.70 (2.45)

-1.08 (2.19)

0.90 (1.15)

FU = Follow up in months SD = Standard Deviation

Number of elbows = number of measurable elbows at that follow-up

* = only one measurement, SD not possible

95 % CI = 95 % Conﬁdence Interval for the accuracy of RSA measurements based on repeated scanning (Valstar et al. 2002)

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humeral components (Table 3). During the ﬁrst two years of follow-up, only one ulnar component rotated more than 1 degree, thereafter migration stabilised.

The radiolucent lines (RLL) at the latest follow-up are summarized in Table 2. Only two ulnar components had RLLs, but these lines were non-progressive during the follow- up.

Discussion

The RSA migration data of the Souter Strathclyde elbow prosthesis show that almost all humeral components migrate up to several millimeters and rotate several degrees in an irregular pattern already at early follow-up moments. This might implicate that initial fix- ation of these components was less optimal and that the mechanical loosening process started during the first postoperative years. The overall migration pattern of the humeral components is in an anterior-varus-external rotation migration pattern. In this pattern the rotatory migration of the humeral component around the y-axis, external rotation, is during the first postoperative year the most progressive continuous migration direction.

Between 1 and 2 year follow-up, an anterior tilt motion of the humeral component is also present. Whether this early movement is typical for the studied Souter-Strathclyde prosthesis was not the focus of this study. However, since results of the long stemmed type of this prosthesis showed superior results to the short stem type in one study, one might argue for improvement of the humeral component.² Contrary, for the ulnar components only one ulnar component rotated more than 1 degree, and none of the ulnar components migrated excessively, these are stable and thus well ﬁxed.³Stable ulnar components were also seen in a RSA study to the Kudo elbow prosthesis by Rahme et al.

Unfortunately, the follow-up was only two years.¹¹

In weight-bearing joints, it has been found by Karrholm et al. for total hip prosthesis and Ryd et al. for total knee prostheses, that increased micromotion at short term follow-up is related to long-term prosthesis survival.^12,13

The end point of prosthesis survival, i.e. usually prosthesis revision, is a less objective measure compared to migration measurements and since every prosthesis will have its own specific migration pattern, absolute migrations values for any prosthesis cannot be generalised to all prostheses. In our publication about the short-term results of this study, we claimed that elbow prostheses at risk for loosening were defined as any increase of translation greater than 0.4 mm or any increase in rotation larger than 1 degree during the second post-operative year.⁸ These assumptions were based on a knee-RSA study.¹³ According to that definition, 8 out of 18 humeral components were at risk for

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loosening at the two-year follow-up evaluation. None of the ulnar components were at risk for loosening. Of the eight elbows that were deﬁned to be ‘at risk’ for loosening of the humeral component at the two-year follow up evaluation, only one turned out to be clinically loose, radiologically and with RSA at long-term follow-up – 121 months – and needed to be revised subsequently. Three other elbows were radiographically loose, but they were not deﬁned ‘at risk’ in the short-term study.⁹ This might be due to two factors.

First, almost all humeral components showed irregular micromotion along the three orthogonal axes, indicating probable “loose” components. However, comparing the revised Souter prostheses with the non revised showed higher MTPM at 2 years follow-up for the former group. Secondly, the low accuracy of detecting RLL along the humeral component’s bone-cement interface. The latter in combination with the small number of prostheses might increase the likelihood of a type II error (lack of power due to the small sample size). Although we see that most humeral components have increased levels of micromotion, the absence of clinical symptoms, even when the prosthesis has loosened, is typical. The same phenomenon is observed in shoulder replacement surgery.

Not all loosened humeral and glenoid components causes clinical complaints, and there- fore revision surgery can be delayed in almost half of the cases.^14,15

The increased levels of migration might be explained by the fact that, the epicondy- lar ridges of the distal humeral bone cannot stabilize the humeral component in a rigid way. Excavating these ridges too much could permit little movements of the cement- prosthesis complex between the medial and lateral pillar. As a consequence the humeral component, and the surrounding cement mantle, can move in these pillars which are not rigidly joined at the diaphysis of the humerus. Even though these motions can be as large as 2 mm, they seem not to be progressive, maybe because of the relative low demand of the joint in daily activities of rheumatoid patients.

Radiological assessments based on convential radiographs in a standardized manner, like the Wrightington method, showed that a prediction of humeral loosening could be made within four years after surgery in this series. Which is at least two to three years later than the real migration of the humeral components. Furthermore this is an under- estimate since all components migrated. The latter is in agreement with others, which that radiolucent lines can be obscured by prosthesis position.¹⁶

At long-term follow-up almost all humeral components, including the ones that were deﬁned to be ‘at risk’ for loosening in our short-term study, were migrating. However, the predictive value of early migration and prosthesis survival at long-term, which was found by others was less obvious in this study due to one factor, end-point deﬁnition in the survival analysis.^12,13 The latter is the major determinant for survival outcome,

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for that matter it is also the most important confounding factor.¹⁷Prosthesis survival is usually deﬁned as reaching the end point prosthesis revision, however since the elbow is mechanically less stressed during activities as compared to hips and knee, a loosened joint will cause less clinical symptoms and thus there will be little reason for revision surgery. The effect of different end-points deﬁnitions on survival analysis has be shown before.¹⁷

Surrogate deﬁnitions of prosthesis performance like survival analysis give some idea on performance however, insight in the underlying mechanism of loosening can only be gained by performing a ﬂuoroscopy study under loading conditions. But, inducible displacement studies of elbow prostheses are not warranted because of the high risk of fracture during these tests.

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Acknowledgments

Piet M. Rozing for the insertion of all elbow prostheses.