L Kerboull, M Kerboull. Marcel Kerboull Institute Imk-forum.com

L Kerboull, M Kerboull. Marcel Kerboull Institute Imk-forum.com
The Charnley Kerboull hip system Results of a 30 years experience in cemented fixation L Kerboull, M Kerboull. Marcel Kerboull Institute Imk-forum.com

basis of the mechanical principles of the cemented fixation
introduction basis of the mechanical principles of the cemented fixation Current Controversies Subsidence and collar Surface finish Cementing technique and cement thickness Classification of cemented stem What must be a perfect cemented stem Clinical Results to support our theory

The original charnley stem : the golden standard
8 % of stem debonding radiolucent line between cement and stem in zone 1 of Amstutz cement mantle crack at the stem tip level even so often asymptomatic, it was for us. a failure of the primary stem fixation

Introduction : Current options
Modern Cementing technique and cement mantle minimal thickness were identified as solutions to address the problem of femoral stem loosening. Our mechanical theory of the cemented fixation was initiated in 1972 and is very different

Taper slip VS Composite beam
Taper-slip system Composite-beam system Flanged Charnley Exeter CPT, C-stem Harris Precoat ? CMK

Mechanical basis of stem cemented fixation
Bone, cement and stem make a composite structure - All mentioned materials have different E-Moduli Cortex: Gpa PMMA: Gpa WHN Stainless Steel Gpa - Each of these materials had different strains under cyclic axial and torsional load that induce micromotion at the interfaces Micromotion primary occurs at the stem-cement interface Micromotion can only be partially absorbed by cement elasticity Micromotion is settled by the stem which is the stiffest component

Mechanical basis of stem cemented fixation
The mechanical stability of this composite Is depending on the mechanical properties and shape of the 3 components But the most rigid component will always have the most positive or negative influence

How to improve stem cemented fixation
Solutions : Improve mechanical properties of cement Increase cement layer thickness Improve cementing technique Improve bone-cement interface Modify stem design to decrease stresses supported by cement Look for a secondary fixation through a distal migration : subsidence Increase link between stem and cement through a rough surface

1 Improve mechanical properties of cement ?
Cement is a viscoelastic brittle material characterized by relatively high compressive strength resistance but weakness in tension and bending Acrylic bone cements: mechanical and physical properties Orthop Clin North Am 2005; 36: 29-39 Kuehn KD and coll Compressive strength 90 Mpa Shear strength 50 Mpa Tensile strength 25 Mpa

Improve mechanical properties of cement ?
« initial migration seems to be independent of the type of cement and of its viscosity » The influence of cement viscosity on early migration of a tapered polished femoral stem Glyn-Jones and coll Int Orthop 2003 ; 27: 362-5 In fact, cement was, is and will ever be the weakest component of this composite structure

2 Increase cement layer thickness ?
We found the answer to this question from the observation of our Charnley first cases Wide medullary canal Thick cement Debonding 36 %

Narrow canal Thin cement Debonding 6 %

Dysplastic femur Very Thin cement 25 y Debonding 0 %

2 Thickening cement layer was not for us the best choice
this observation suggested that a stem fitted to cortical bone with a thin cement layer might improve the cemented fixation It was also evident that an undersized stem used to get a thick cement mantle was not the good solution to prevent distal migration, because even thicker the cement was not enough resistant to face the stresses transmitted by the stem. « initial migration seems to be independent of the thickness of the cement mantle » Influence of cement viscosity and cement mantle thickness on migration of the Exceter total hip prosthesis Nelissen RG and coll J Arthroplasty 2005;20:521-8

Solutions : Improve mechanical properties of cement Increase cement layer thickness Improve cementing technique Modify stem design to decrease stresses supported by cement Improve bone-cement interface Look for a secondary fixation through a distal migration : subsidence Increase link between stem and cement through a rough surface

3 Improve cementing technique ?
Why : To increase resistance of cement to bone interface through a deep penetration of cement to create interdigitation To get an homogeneous and more resistant cement mantle At least 3 mm

3 Improve cement and cementing technique ?
How ? : Low viscosity cement Vacuum preparation to avoid air High Pressure water cleaning High pressure cement insertion Stem tip centralizer undersized stem

What can we expect from these techniques ? Prevent early distal migration ? Definitely no Prevent late loosening No influence for the shaped closed fixation, like CMK May be for the loaded-taper fixation, like Exceter But with a time consuming, aggressive for the bone and demanding technique With an important increase of the overall price of the proceedure

Our current routine technique Plugging and simple washing of the medullary canal Use of a Standard viscoelasticity cement Simple Cement insertion using a syringe High Pressure is applied by the canal filling stem So for us Cementing technique is not a main issue And must remain a friendly and simple technique

4 Improve bone-cement interface
Bone is twin : cortice and cancellous Does cancellous bone is able to carry the load ? NO : Ebramzabeh E and coll : the cement mantle in total hip arthroplasty : analysis of long term radiographic results JBJS A,77-87 Effect of aging: lowering of mechnical properties of cancellous bone Are Interdigitations between cement and cancellous bone necessary ? Yes if you use a force loaded stem that submit the cement to high tensile stress No, if cement is only submitted to a low level of compressive stresses by a canal filling stem cemented line to line

Further under loading and aging cancellous bone undergoes compression and becomes uneven In this situation, the cement mantle is subjected to bending and tensile stresses and will crack. So, removing the cancellous in the superomedial part of the metaphysis and in the distal canal gives the cement an even and rigid base and prevents its crack under bending stress

Removal of cancellous bone which is not able to carry load with aging Modification of the bone bed preparation Femoral Reamer Instead of broach Less agressive More precise

Mechanical basis of the stem debonding
High bending stresses in the supero medial part break the cement mantle This rupture widens the proximal part of the cement mantle decreases the shear stresses along the stem increases the vertical force on the distal cement which breakes under tensile stress and finally allowed varus tilt and subsidence of the stem

5 Modify stem design : WHY ?
Because it was for us the most logical way: to decrease the level of stress within and at the cement stem and cement bone interfaces to only subject the cement mantle to compressive stresses and consequently avoid the problems of the cement layer thickness and resistance

Were retained from the Charnley stem
5 Modify stem design Were retained from the Charnley stem Polished surface Ra 0.04  (1.6  inch) Collar Rectangular cross section The cup design

Three main modifications of the stem design resulted in the MK 1
5 Modify stem design Three main modifications of the stem design resulted in the MK 1 Opening the stem-neck angle to 130° instead of 125° widening the proximal part of the stem Increasing the range of sizes

Stem CCD angle and cement loading
5 Modify stem design Stem CCD angle and cement loading

Opening the stem neck angle to 130°
5 Modify stem design Opening the stem neck angle to 130° to decrease the pressure on the supero medial and infero lateral part of the cement mantle CCD angle 125° 130° Charnley Kerboull

Widening and thickening the stem proximal part
5 Modify stem design Widening and thickening the stem proximal part to give it a double tapered shape with a cross-section sufficently decreasing (taper angle > 5°) so that

Widening and thickening the stem proximal part
5 Modify stem design Widening and thickening the stem proximal part the shear stresses along the stem would be progressively transformed into their pressure components and the vertical distal force would be dramatically reduced

5 modify the stem design: A large range of sizes
1 to reconstruct a normal architecture in every case (limb length and abductor muscles off-set).

2 to get a self alignment of the stem with the femoral diaphysis axis “ in our study, line to line stem without distal centralizer were better aligned than undersized stems fitted with a centralizer” T Scheerlinck and coll CT analysis of defects of the cement mantle And alignment of the stem JBJS 88 B,

A large range of sizes Symmetrical anatomy

to have between stem and femoral canal the best fit to reduce load transmitted to ciment layer

Under these conditions
Cement mantle, cement bone interface are no longer subjected to shear stresses and micromotion is reduced to a level tolerated by creep of cement With this canal filling stem, the double tapered shape is acting to decrease the stresses on the cement, but with an undersized stem the distal force increases and the stem subsides

According to the preoperative planning
How to choose the appropriate size regarding to the canal filling concept According to the preoperative planning Not the largest one that will impose to ream the cortices but The first size that get self alignment primary stability

How is the cement mantle thickness around a CMK stem
Ant. Post. Relatively thick > 2 mm Med. Lat. Thin or very thin < 1 mm uneven But never incomplete T Scheerlinck and coll CT analysis of defects of the cement mantle And alignment of the stem JBJS 88 B,

Uneven cement mantle but never incomplete
And protected of overloading by the stem design

How to improve the cemented stem fixation ?
If you consider subsidence as a positive event to lock the fixation of the stem in the cement mantle you will try to favourize it but you will need to reinforce the cement to lower the distal migration If you consider subsidence as a primary failure of the initial fixation you will try to protect cement of over loading to prevent distal migration and to protect the ciment the only possibility you have is to work on the stem design

Subsidence and collar ? Why ?? Is Subsidence a normal event ?
- probably No for the CMK which is not designed to subside and does not subside. « The French paradox »: Langlais, Ling, Kerboull, Sedel. JBJS Br, 2003. Why ?? - We choose the stem which best fills the medullary canal there is no space for the stem to subside. - The stem does not subside due to the cohesion forces acting on the two polished (cement/stem) surfaces and micromotion stays under its cement fracture level The collar may decrease the distal force applied to ciment plug does not prevent migration if it occurs just an intraoperative reference to set leg length

Does CMK subsides ? Long-Term Migration Using EBRA-FCA of Stems Cemented Line-to-Line According to the "French Paradox" Principles Hamadouche, Moussa; Kerboull Luc; Kerboull, Marcel ORS 2008 The EBRA-FCA software is a validated method designed to assess migration of a femoral component through comparable pairs of radiographs. Accuracy has been reported to be better than ± 1.5 mm (95% percentile), with a specificity of 100% and a sensitivity of 78% for the detection of migration of more than 1.0 mm, using RSA as the gold standard

Mean subsidence of the entire series was 0.63 ± 0.49 mm
Does CMK subsides ? Materials and Methods: In 1988 and 1989, 164 primary THA in 155 patients by the two seniors of us. mean age ± 11.6 years. Polished CMK Results: 73 patients (77 hips) still alive F.U 17.3 ± 0.8 years (15-18 years), 8 patients (8 hips revised for high polyethylene wear 66 patients (69hips) deceased 8 patients (10 hips) lost to follow-up. 1689 radiographs (mean 10.3 per hip) were digitized. 263 (15.6%) excluded No migration curve obtained for 22 of the 164 femoral components (13.4%). Mean subsidence of the entire series was 0.63 ± 0.49 mm Using a 1.5 mm threshold for subsidence, 4 of the 142 stems have migrated. Using a threshold of 2 mm for subsidence, none of the 142 stems have migrated.

does not subside up to 18-year follow-up.
Does CMK subsides ? NO this study demonstrates that contrary to other cemented femoral components that have also provided excellent survival in the long term but frequently associated with stem subsidence, The CMK Stem, a highly polished double tapered femoral component with a quadrangular cross-section and a collar, filling the medullary canal, and cemented with a simple technique does not subside up to 18-year follow-up.

What is Subsidence ? Subsidence below 2 mm may be probably absorbed by cement creeping and contributes to lock the stem but overloads the cement Subsidence over 2 mm always induces a fracture of the cement mantle and is a loosening that may be tolerated Because distal migration without varus tilt is often well clinically tolerated that might explain the good survival of taper-slip stem if only revision is considered as a failure.

Solutions : Improve mechanical properties of cement Increase cement layer thickness Improve cementing technique Improve bone-cement interface Modify stem design to decrease stresses supported by cement Look for a secondary fixation through a distal migration : subsidence Increase link between stem and cement through a rough surface : surface finish

Why the polished stem became matt ?
1 Initiation of stem loosening: debonding of the cement to prosthesis interface Improvement of the bond through a matt surface Composite beam concept

Why the polished stem became matt ?
Manufacturer request to facilitate production 2

Matt stem (CMK3) Vecteur Orthopédique
Polished stem (MKIII) Stryker Howmedica Ra = 0.4mm Ra = 3mm Quadrangular cross-section Oval cross-section

Stem Radiological loosening
Polished stems: hip (revised) related to PE socket wear Matt stems: 16 hips (9 revised) 3 related to PE socket 13 debonding at the bone cement interface associated to femoral osteolysis

Matte stems loosening (CMK3)
1y 3y 5y

Matte stems loosening (CMK3)

Femoral stems survival with radiologic loosening as the end-point
97.3% 78.9% Log-rank test, p < 0.001

DISCUSSION Significant lower survival for matt stems Similar observations made with other stems designs: - Exeter stem: Howie et al., JBJS Br, matt VS 40 polished stems, Minimum 9 year-FU 4 matt stems loosened, 0 polished stem - Iowa Stem: Collis et al., JBJS Am, grit-blasted VS 122 polished stems, mean 5,8 year-FU 6 gritt-blasted stems loosened, 0 polished stem

VS What was our main mistake Surface finish change
cross section change ?

micromotion, settled by the stem, occurs
do not forget Micromotion Fact: - Bone, cement and stem form a composite - All mentioned materials have different E-Moduli Cortex: Gpa PMMA: 1.8 Gpa WHN Stainless Steel 250 Gpa - As all material deform differently under load micromotion, settled by the stem, occurs

SO ……… With a Matt surface PMMA Stem Bone
No relative movement between stem and cement possible Micromotions result in localy debonded areas at the bone cement interface which create an abrasive medium that induce osteolisys Bone PMMA Stem

While a polished surface (associated with a good design)
micro movements occurs at the stem-cement viscoelastic interface and the bone-cement interface is protected Bone PMMA Stem

We conclude that - An increased cement-prosthesis bond - increases shear stresses at the cement-bone interface - and finally induces loosening at the bone-cement interface

Stems are prone to relative rotation when torque forces are applied
Round or Rectangular cross section ? Fact : Stems are prone to relative rotation when torque forces are applied

Round vs Rectangular : peak stresses
Rounder cross section Prevents peak stresses at the angle But Increased shear stresses at the interface Rectangular cross section Might induce peak stresses Applies compressive stresses on the cement M2 M1 shear stress compressive stress F1a F1b a b

cross section design : delicate choice
Too round results in problems Too edgy might result in problems (increased hoop stresses) Look for a compromise Almost rectangular with smooth angles

Results of the first generation of CMK

Charnley: Radiological aseptic femoral loosening
220 hips 100% 90% 80% 74% 70% 60% 50% 5 10 15 20

Long term results : elderly patients
RCO, 1995

Long term results : patients < 40 years old
European Journal of Orthopaedic Surgery and Traumatology 1996, 6,

Long term results : CDH Journal of Arthroplasty 2001, 16, N°8

Charnley vs CMK 14 Y 30 Y

Longterm features

No cement under the tip

Dysplastic stems and acetabular reconstruction
20 years

Results of the next generations of CMK

Long term results in patients under 50 CORR, 418, Jan 2004ries
287 THR performed from 1975 to 1990 Randomly sorted from a cohort of 2804 patients Senior and junior surgeons 222 patients, 144 females and 78 males Mean age : 40,1 y ( ± 8 y ; 15, y). Mean weight : 63 kg (± 18,2 kg ; 37 – 116 kg) 287 primary total hip replacement done in 222 patients were chosen randomly from a cohort of 2804 patients younger than 50 years. Strtification using software was done according to the year of the surgical procedure. Mean age of the patients was 40,1 years. The mean weight was 63Kg and 80% of the patients had a normal boby mass index. Physical activity was important only in 115 hips. A large majority of tha patients had no other associated pathology.

Mean follow-up : 14,5 years (± 5.1, 6m to 25 y)
Status Mean follow-up (years) Number of hips Reviewed 16,1 ± 4,6 210 ( 73,2%) Died of unrelated causes 5,4 ± 3,1 10 (3,5%) Lost to follow-up 10,8 ± 5,1 42 (14,6%) Revised 12,6 ± 6,1 25 (8,7%) 210 hips were reviewed at a mean of 16,1 years posoperatively. 10 hips belonged to 1O patients died of unrelated causes at a mean of 5,4 years. 42 hips were lost to follow-up at a mean of 10,8 years and 25 were revised at a mean of 12,6 years. The mean follow up of the entire series was 14,5 years and 52 hips had a follow-up greater than 20 years. 52 hips had a follow-up greater than 20 years

Radiological results : Stem loosening
No loosening Definite or probable loosening Potential loosening Number of hips 271 95,1% 12 4,2% 4 0,7% On the femoral side according to the criteria used in this study, 271 stems were not loosened, 12 stems had a probable or definite loosening and 4 had a potential loosening. Among the 12 definively stems 11 had subsided of 3 to 5 mm. One of these loosening was septic and one other postraumatic. Seven of these stems where revised. 10 aseptic, 1 septic, 1 after a periprosthetic fracture

Radiological loosening : stem
Implants design Surface Section Aseptic loosening Hips MK I Polished Quadrangular ,7% 139 CMK 2 Matte Oval 2 4% 51 MK III 27 CMK 3 8 11,5% 70 The femoral component used in this series were available in four configurations. Two types were polished and two types were mate. Polished implantswith a quadrangular section had a definite loosening of 0,6% 1 hip of 166 whereas matte implants with an oval section had a definite loosening rate of 7,5% , 9 hips of 121 hips. This difference was significant. Matt vs Polished : p = 0,0001

Survival rate : revision for any reason
1 20 years 85,4% 95% sup : 92,4 % 95% inf : 78,4 % ,8 Suvival rate % ,6 ,4 ,2 The survivorship analysis with revision of either component for any reason as the end point yielded a 85,4% cumulative survival rate at 20 years. The survival rate of the acetabular component was 93,6% and the survival rate of the femoral component was 95,8% 5 10 15 20 25 30 Follow-up years

Survival rate : stem loosening, surface finish
P= 0,0001 ,2 ,4 ,6 ,8 1 5 10 15 20 25 30 96,4% 88,5% Suvival rate % polished matt Follow-up years

Survival rate : PE wear dependant
Suvival rate % Wear > 0,1 70,6 % P = 0,002 As a demonstration of the influence of wear on the survival rate, hips with normal wear exhibited a survival rate of 94,8% whereas hips with abnormal wear had a 70,6% survival rate at 20 years. Follow-up years

What we learnt at the end of this study
The Kerboull cemented prosthesis could provide satisfactory and durable results for 20 years in 85 % of patients younger than 50 years. The stem has a reliable fixation without subsidence and with a simple surgical technique So in our mind it remains a good solution and reliable solution even for young patient

What we learnt at the end of this study
Stem design and surface finish are the most important factors. Surface must be definitely polished to protect bone-cement interface from overloading. Cement is weak, and must be protected by the stem of excessive tensile and shear stresses, and just used to fill the gap between the bone and a canal-filling stem. Following these mechanical principles cement quality, cement thickness and cementing technique become minor concerns Be careful, the classifications and terminology used in the literature are confusing because they mainly refer to the cementing technique and not to the design and the surface The CMK is a loaded taper but initially stable due to its fitting to the medullary canal

Thank you for your attention
Trust the stem design more than the cement mantle． Thank you for your attention

L Kerboull, M Kerboull. Marcel Kerboull Institute Imk-forum.com

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