Design and application of combined hip-knee intramedullary joint replacements

The Journal of Arthroplasty Vol. 14 No. 8 1999

Design and Application of Combined Hip-Knee Intramedullary Joint Replacements

P e t e r S. W a l k e r , P h D , * W a i W e n g Y o o n , BSc ,* S t e p h e n R. C a n n o n , F R C S , t

G e o r g e B e n t l e y , F R C S , { a n d S a r a h K. M u i r h e a d - A l l w o o d , F R C S §

Abstract" Cases in which there is a total hip arthroplasty and a stemmed total knee arthroplasty in the same femur, with loosening of 1 or both components, with serious endosteal bone loss or even a fracture between the stems present a difficult reconstruction problem. We describe a reconstruction using a combined hip and stemmed knee, designed so that they could be rigidly connected during the surgical procedure. The advantages of this implant design are that the entire femur with its muscle attachments is preserved, and the inherent stability allows for immediate weight bearing. To determine the viability of the connection between the hip and the knee, a stress analysis was carried out using finite element analysis. Guidelines were thus provided for the required metal and cement thicknesses. Three case examples are presented with an average-follow-up of 3 years. It was shown that the combined hip-knee implant could provide successful results for these difficult reconstructive problems in appropriately selected cases. Key words: combination hip-knee total joint, hip arthroplasty, knee arthroplasty, massive prostheses, finite element analysis.

Hip and knee rep lacement within an ipsilateral f emur is an increasingly recognized or thopaedic occurrence. Revision for loosening of ei ther of these components can lead to difficult reconstructive problems, which may be associated with loss of bone stock and fracture. This reconstruct ion can be particularly difficult if a knee rep lacement wi th an in t ramedul lary s tem has been previously used. The loosening process usually results in loss of endosteal bone, which m a y be, on occasion, severe, and the s tem of ei ther hip or knee can penet ra te the cortical area of bone leading to a stress-riser and fracture. It

From the *Centre for Biomedical Engineering, University College London, Royal National Orthopaedic Hospital Trust, Stanmore; ~Royal National Orthopaedic Hospital Trust, Stanmore; ~Institute of Orthopae- dics, Royal National Orthopaedic Hospital Trust Stanmore; and §The Whittington Hospital, London, and Royal National Orthopaedic Hospi- tal Trust, Stanmore, United Kingdom.

Submined September 9, 1998; accepted April 26, 1999. No benefits or funds were received in supporl of the study. Reprint requests: Peter S. Walker, PhD, Centre for Biomedical

Engineering, Universi ty College London, RNOHT Stanmore, Middlesex HA7 4LP, UK.

Copyright © 1999 by Churchill Livingstone ® 0883- 5403/99/1408-0009510.00/0

has been well recognized that in hinged knee replacements in which the stem tip is initially adjacent to the endosteal surface, loosening and fracture frequently occur [ 1 ]. It is standard or thopaedic practice to extend fixation of revision steins beyond the area of bone destruction and endosteal resorption to obtain sufficient fixation of the revised implant. If both hip and knee replacements are present, this practice can lead to close proximity of the revision stems and increase the chance of a fracture of the bone. When in t ramedul lary stems are closely applied, any revision situation does not allow increased length to be gained. In cases in which fracture has occurred, conservat ive treat- men t is usually preferable [2,3], but it is well recognized that in the presence of poor bone stock, particularly w h e n the endosteal surface is covered with cement , fracture healing is unpredictable, and fixation of external plates, either by screws or by circumferential cramping, is fraught with difficul- ties. The older pat ient may be unable to endure prolonged periods of n o n - w e i g h t bearing or bed rest that may be required to obtain healing.

945

946 The Journal of Arthroplasty Vol. 14 No. 8 December 1999

The combined h ip -knee in t ramedul lary joint re- p lacement was designed to address the a foremen- t ioned situations by providing immedia te stability, allowing early mobilization. The implant consists of hip and knee stems that are rigidly connected at surgery. The aims of the present study were to de termine the viability of the implant regarding the strength of the connect ion be tween the stems, the preservat ion of the bone, and the functional possi- bilities. A stress analysis was carried out to determine guidelines for the dimensions of the stems and cement mantle, and the clinical sequelae are illus- trated from fol low-up of 3 patients. This article presents a shor t - te rm analysis that concludes that the mechan i sm is sound and will remain intact in the short- term. To date, we have not recognized any major problem with bone resorption or loosening, but a longer fol low-up is required to address these problems. This article therefore represents an early report outlining the feasibility of the method.

Mater ia ls and Methods

Implant Design and Stress Analysis

The femoral componen t was cus tom-made to be a close fit to the canal in the proximal femur. This region was grooved, grit blasted, and hydroxyapatite coated. The distal part of the s tem was smooth. The in t ramedul lary stems of the knee hinge were also t i tanium alloy, again grit blasted and hydroxyapatite coated for uncemen ted applications. The knee hinge-bearing componen t s were made f rom coba l t - ch rome alloy and u l t r a -h igh -molecu la r - weight polyethylene.

A three-dimensional finite e lement model was formulated based on a typical design from the clinical cases (Fig. 1). The dimensional parameters were the s tem diameter (10, 12, and 14 mm) , the cement mant le thickness (2, and 4 ram), and the wall thickness of the outer tube (1, 2, and 3 ram). Analyses were carried out for all combinat ions of these parameters , which were chosen to be within a feasible range to fit typical femora. One fur ther model was used to investigate the effect of debonding on the c e m e n t - m e t a l interfaces.

The finite e lement package used was COSMOS, which was capable of linear and nonl inear solu- tions, the latter being used to investigate the effect of debonding of interfaces. The properties of the materials were t i tanium alloy, modulus of elasticity E = i06,000 MPa, Poisson's ratio v = 0.33; acrylic bone cement, modulus of elasticity E = 2,700 MPa, Poisson's ratio v = 0.33. The materials were assumed to be homogeneous , isotropic, and linearly elastic.

A

FRONTAL

AXIAL FORCE

stem 0,ameter

cement thickness

wall - - ~ .~ thickness

acrylic ~ ~ cement

tubular part - " " of knee

femoral component ~ " acryl ic

spacer "1 (optional) > .........

) M

base fixed

Fig. 1. (A) A schematic of the combined hip-knee intramedullary joint replacement. The model of the hip-knee connection is shown for the finite element analysis. (B) The finite element model showing the quadrilateral elements. The white elements represent the cement.

Combined Hip-Knee Replacements • Walkeret al. 947

Regarding the forces and moments that occur in the diaphysis of the femur, data have become available from a telemetrized distal femoral replacement [5,6]. For level walking, the axial force was 1,500 N, whereas the bending moments in the frontal and sagittal planes peaked simultaneously at 50 and 30 N. m. These values were measured at 230 m m from the distal end of the femur. The level of interest for our h ip-knee implant is at the connection between the stem and the tube, 120 m m being representative. Assuming the moments re- duce linearly to 0 at the knee [4], the resultant m o m e n t at the 120-mm level was calculated to be 30 N . m . Hence, a 1,500-N axial force and a 30-N - in momen t were applied to the model.

Eight-noded quadrilateral elements were used. To obtain a satisfactory mesh, analyses were carried out, then the mesh density was progressively increased in the regions of maximal stress gradient, maintaining a satisfactory aspect ratio of the elements, until convergence of the results. The final models consisted of 11,000 elements.

component was designed for proximal cementation, cement was placed in the proximal region using finger packing after the stem had been inserted partway. Cement was placed inside the tube of the knee component as well as around the distal end just before final seating. This component was then pressed into place. Care was taken at the stage when the tube engaged the tip of the hip componen t to ensure it was central and fully bot tomed out. The tube itself had small acrylic spacers fixed to the inner wall for stem centralization. After the cement had hardened, the hip was reduced. The tibial component and knee assembly was then carried out. The assembly was finally checked at both the hip and the knee before closure.

Radiographs and clinical examinat ion were carried out postoperatively and at 6 months, 1 year, and yearly after that. General clinical observations were recorded. The radiographs were examined for radiolucent lines around the interface and for any signs of change in bone density or movemen t of the components within the bone.

Surgical Technique and Clinical Analysis

The technique varied to some extent for each specific case, but there were certain general features. The exposure was as if performing simulta- neous hip and knee replacements. The existing components were removed together with all rem- nants of cement and fibrous tissue adjacent to the endosteal bone. The new hip and knee femoral components were introduced separately to test for access, and further bone was removed as required. Attention was given to ensuring that the hip femoral component centered in the canal distally. This centering was especially important if the proximal end of the componen t was close fitting for unce- mented use (hydroxyapatite coated). The acetabu- lar component was revised, if necessary. The hip componen t was then placed, and trial femoral heads were used to ascertain that the hip was stable. The total knee was assembled and checked for range of motion and patellar tracking.

The hip and knee components were then both inserted in that order, and the leg length was measured. In cases in which there had been a midfemoral fracture, restoration of length was necessary. This restoration could be accomplished by placing acrylic spacers inside the tube of the knee component . Such spacers were also used when the length was correct, to provide a positive seat when the components were being cemented. The canals were then lavaged before final fixation of the components . The hip componen t was inserted first, again checking for distal centralization. When the

Results

Stress Analysis

The results were computed as maximal tensile and compressive stresses, so that the values could be compared with k n o w n strength values of the materials [7-10]. The tensile stresses in the stem as a result of bending were at a max imum where the stem entered the cement annulus. The tensile stress for a diameter of 10 m m was found to be less than 60% of the fatigue limit of t i tanium alloy with the larger-diameter stems showing even lower values. The stresses were minimally affected by increasing the cement or outer tube thickness.

The cement stresses are shown in Fig. 2 for a range of variables. As expected, the stem diameter was a major factor, with a 10-mm diameter produc- ing stresses that were excessive in both tension and compression. There was clearly an advantage in maximizing the stem diameter, but only at a 14-mm diameter did the stresses become lower than the fatigue limit. A thicker cement mantle reduced stresses significantly. The compressive strength of cement is likely to be the most important criterion here, as discussed later.

The stresses in the outer tube were less than one half those in the stem up to stem diameters of 14 m m and hence would not be a strength concern. Increasing tube wall thickness > 1 m m reduced tube stresses, but this led to an increase in the cement stresses by increasing the rigidity of the composite.


4 0 -

Z O 3 0 - Go z 20 LU

10

MPa

1 0 - Z O 20 - GO Go 3 0 - LU n" n 40 -

O 5 0 - O

CEMENT MANTLE THICKNESS

~ 2 m m ~ 4 m m

TENSILE - - ~ STRENGTH

~ . ~ (Burke)

FATIGUE LIMIT (Krause)

10 mm S "~'~' ULTIMATE COMPRESSIVE STRENGTH (Saha)

Fig. 2. The maximal tensile and compressive stresses in the cement annulus for stem diameters of 10, 12, and 14 mm. Cement mantle thickness, 2 and 4 ram; outer tube wall thickness, 1 mm.

The increases in cement stresses were relatively small, however , and did not bring the cement stresses close to the compressive strength.

The analysis of the unbonded c e m e n t - s t e m interface showed an increase in stem tension and com-

pression of 12% and 10%, with similar changes in the stresses of the outer tube. The cement stresses were changed the most, with a reduct ion in tensile stress to almost 0, and an increase in compressive stress of 20%.

Cl in ica l C a s e s Case 1. A 36-year-old w o m a n with rheumato id

arthritis presented with a fractured femur above the femoral stein after a hinged total knee ar throplasty (Fig. 3A). Initial t r ea tment had involved double AO (Synthes, Robert Mathys Co., Bettlach, Switzer- land) plating and bone grafting, but she went on to have a significant nonun ion with varus deformity of the hip. The hip was severely affected by rheuma- toid arthritis and was stiff in all modalit ies of m o v e m e n t . It was decided to treat the pat ient with a combined h ip -knee replacement . A femoral component of hip rep lacement was designed whose proximal half was coated with hydroxyapat i te and designed to press-fit into the femur. The distal part of the stein was grooved to allow cement fixation within the hol lowed c o m p o n e n t of a SMILES rotating hinge knee rep lacement (Stanmore Implants Worldwide, Ltd., UK). The femoral componen t of the hip rep lacement was inserted first, then f rom the distal f emur cement was inserted a round the distal port ion of the hip stem, and the SMILES rotating hinge femoral c o m p o n e n t was cemented onto the stem. The pat ient had immedia te stability

Fig. 3. (A) Preoperative view of a 36-year-old woman shows an unsuccessful attempt to fix fracture (case 1). (B and C) The combined hip-knee implant, using a SMILES rotating hinge (case 1 ).

Combined Hip-Knee Replacements • Walkeret al. 949

in the postoperat ive period and was able to mobilize in a fully weight-bear ing manner .

Radiolucent lines were seen immedia te ly postoperatively a round part of the knee s tem because of the condit ion of the canals after the revision of the knee hinge, but these lines were nonprogress ive (Fig. 3B and C). The u n c e m e n t e d long-s t emmed total hip rep lacement appeared stable wi th no evi- dence of radiolucent lines or osteolysis proximally, despite the poor femoral bone stock. The pat ient had satisfactory funct ion 3 years after insertion.

Case 2. A 61-year-old man with rheumato id arthritis suffered a loosening of a left total hip rep lacement above a loose left S tanmore total knee rep lacement (Biomet, UK). As a result of excessive micromot ion, a fracture occurred be tween the 2 components . An initial stabilization was a t t empted using a M e n n e n plate (C.H. Medical, Exeter, UK) externally, but this failed rapidly (Fig. 4A). A combined revision of the hip and knee was decided on. On this occasion, a cemented femoral c o m p o n e n t for the hip rep lacement was used. Cemen t was inserted only after initial stabilization of the stem into the diaphysis of the femur. Once the cement a round the hip rep lacement had cured, a fur ther cemented technique for the femoral c o m p o n e n t of the knee rep lacement was per formed from the distal end of the femur.

No radiolucent lines were seen a round the interface of the hip s tem (Fig. 4B). The distal femoral

c o m p o n e n t displayed a radiolucent line because of the lack of t rabecular bone on the revised endosteal surface (Fig. 4C), but the line did not progress 15 mon ths postoperatively. Overall, there was no sign of progressive loosening at 15 months , and the pat ient was satisfied with the prosthesis f rom a functional point of view.

Case 3. A 64-year-old w o m a n with rheumato id arthritis unde rwen t revision of a hip rep lacement with a long-s t emmed cemented prosthesis. During the initial postoperat ive period, a fracture of the distal two thirds of the femur occurred. Bone grafting a t tempts were made, but she was left wi th a significant nonun ion and poor bone stock. The knee benea th the fracture was ex t remely stiff and showing features of rheumato id arthritis (Fig. 5A). On this occasion, the stable cemented hip c o m p o n e n t was left in place and a custom-bui l t femoral componen t was designed to allow cementa t ion onto the s tem in situ. Immedia te stabilization of the construct was noted, and the patient was able to mobilize in a weight-bearing manner . On this occasion, an adapted Kinemax total knee replacement (Stryker-Osteonics- Howmedica, Rutherford, N J) was used (Fig. 5B).

Radiographically, there were no signs of loosening or migrat ion of the knee or the hip c o m p o n e n t 4 years after the initial procedure. On examina t ion at 4 years ' follow-up, the pat ient cont inued to walk well wi thout requiring any cane or crutch. The

Fig. 4. (A) A Mennen plate used unsuccessfully to stabilize a fracture between the hip and knee stems (case 2). (B) Proximal cementation of the femoral hip stein (case 2). (C) The distal region using a SMILES rotating hinge (case 2).


Fig. 5. (A) Preoperative anterior view of a distal fracture around a long-stem hip (case 3). (B) Postoperative anterior view showing the same femoral component and a modified Kinemax condylar (case 3).

range of flexion at the knee was 95 ° , and the pat ient was satisfied with the outcome.

Discussion

In general, w h e n designing custom implants, it is not feasible to carry out a complete analysis of each one because of the time and expense involved. Design guidelines can be formulated, however, which general- ize the problem and which can then be applied to each case. In this case, stresses in the implant components are the most important consideration to avoid a mechanical failure. Calculated stresses can be compared with the mechanical propert ies of the materials, the fatigue stress being the most relevant. The properties of materials such as acrylic cement and polyethylene vary depending on processing methods [7-9], and bone properties vary over a wide range, whereas defects in the cortex are c o m m o n in this type of case with unknown stress-concentrating effects.

Another p rob lem in the analysis is that there are inadequate data of the forces acting on the bones and on the components . Al though the forces on the hip itself have been determined directly by telemetry, knee forces have been de termined only by indirect calculations. Estimates of the forces on implant componen t s need to account for the level in the femur, on the muscle forces be tween the hip or knee at the required level [4-6], and the mechanical

connect ion (such as cemented) be tween the component and the bone. In the h ip -knee implant, te leme- trized data were used of the forces and m o m e n t s acting on the shaft of a distal femoral replacement . In translating these data to the required level, it was assumed that the axial force would be constant in the shaft, which would occur for no s t e m - b o n e bonding. The frontal and sagittal m o m e n t s were calculated to diminish f rom the hip to the knee, being small at the knee itself [4].

W h e n the stresses that were calculated were compared with the material properties, it was clear that the cement was most at risk. To minimize cement stresses, the ideal dimensions are 14-ram diameter for the stem, 4 - m m cement thickness, and 1 -mm tube wall thickness, giving a total of 24 m m outer diameter. The available bone canal diameter, however , is usually less than this, requiring compro- mises to be made. A fur ther detail is that the s tem centralizers in the metal tube are impor tan t to avoid a region of small cement thickness.

The indications for combined h ip -knee implants require careful consideration, and more conserva- tive approaches mus t be considered first. Such an implant is appropriate, however , if revision of a loose hip or knee would have a poor prognosis or if it were not possible to obtain fracture healing be tween a hip and knee stem. Severe endosteal bone loss, possibly after previous revisions or f rom wear particles, is also an impor tan t consideration pointing toward a h ip -knee implant. There are several positive advantages, in particular, the implant allows early weight bearing, and future loosening in the femur is no longer an issue. The surgical procedure m a y only require incisions similar to those for a total hip and total knee replacement , but a midthigh incision m a y be needed in cases in which there had been a midshaft fracture.

The possibility of removal of a h ip -knee implant must be considered in case of infection. For this reason, it is an advantage to use cement or hydroxyapatite coating close to the hip and the knee only and not along the entire length of the femur. Stress shielding of the femur is an issue, although if the implant is fixed at the hip and knee regions, both axial and bending forces are transmitted to the femur. In addition, the muscles themselves apply significant forces. Considering the severity of the predisposing condition in which the h ip -knee implant is used, the gain in function is likely to be sufficiently impor tan t to justify the risk of some osteopenia. In any case, we believe the h ip -knee implant is preferable, in most situations, to one alternative, which is a total femoral replacement , owing to the preserva- tion of muscle and soft tissue a t tachments .

Summary

A c o m b i n e d h i p - k n e e i m p l a n t can be d e s i g n e d to h a v e suff ic ient s t r eng th . The surgical p r o c e d u r e is feasible, a n d the poss ib i l i ty of f u n c t i o n a l r e s t o r a t i o n has b e e n d e m o n s t r a t e d . It is sugges t ed tha t t he h i p - k n e e i m p l a n t is a use fu l t r e a t m e n t for t he se r ious p r o b l e m s desc r ibed .

Acknowledgments

We a c k n o w l e d g e Ms. S. S a t h a s i v a m for ass i s tance r ega rd ing the ana ly t i ca l p r o c e d u r e s , a n d Ms. C. C o a k l e y for co l l a t ion of t he med i ca l records .

References

1. Inglis AE, Walker PS: Revision of failed knee replacements using fixed axis hinges. J Bone Joint Surg Br 73:757, 1991

2. Bethea JS, DeAndrade JR, Fleming EL, et al: Proximal femoral fractures following total hip arthroplasty. Clin Orthop 170:95, 1982

Combined Hip-Knee Replacements • Walker et al, 951

3. Jensen JS, Barford G, Hansen D, et al: Femoral shaft fracture after hip arthroplasty. Acta Orthop Scand 59:9, 1988

4. Duda GN, Schneider E, Chao EYS: Internal forces and moments in the femur during walking. J Biomech 30:933, 1997

5. Taylor S J, Walker PS, Perry J, et al: The forces in the distal femur and knee during different activities measured by telemetry. Trans Orthop Res Soc 22:259, 1997

6. Taylor S J, Walker PS, Perry J, et al: The forces in the distal femur and the knee during walking and other activities measured by telemetry. J Arthroplasty 13: 428, 1998

7. Krause W, Mathis RS: Fatigue properties of acrylic bone cements: review of the literature. J Biomed Mater Res 22(suppl A1 ):37, 1988

8. Lee AJC, Ling RSM, Vangata SS: Some clinically relevant variables affecting the mechanical behaviour of bone cement. Arch Orthop Traumat Surg 92:1, 1978

9. Saha S, Pal S, Albright JA: Time dependent mechanical properties of normal and carbon fiber reinforced bone cement. Trans Orthop Res Soc 6:296, 1981

10. Brochure. IMI Titanium Ltd, Birmingham, UK

Documents

Design and application of combined hip-knee intramedullary joint replacements