Abstract
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The French paradox cementing technique encompasses a canal filling highly polished stem with a thin (<1 mm) cement mantle.
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The technique has been developed by Pr Marcel Kerboull in the late 1960s after he observed the patterns of debonding of the original Charnley stem.
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The key point of the technique is based upon removal of the metaphyseal cancellous bone (with hollow reamers or aggressive broaches) especially at the supero-medial region.
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Only two stems have been validated with this technique: the Charnley–Kerboull (CK) and the Ceraver Osteal stem, both of which are collared.
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This technique is neither a taper slip (the stem does not subside at long-term follow-up) nor a composite beam (a highly polished stem is used).
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A 12% shortened stem CK has shown similar results to the standard-length stem, including the absence of stem subsidence.
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Combined with the Hueter anterior approach, this technique has demonstrated one of the lowest femoral PPF rate in elderly patients in the literature.
Introduction
Sir John Charnley defined modern total hip arthroplasty (THA) in the 1960s (1). With his idea of using bone cement (poly-methyl methacrylate, PMMA) to fix the femoral component to the bone, he discovered a fixation method that later proved to be the gold standard (2).
Despite the subsequent development of cementless stems, cemented stems are regaining interest in the orthopaedic community, as this mode of fixation has a protective effect against periprosthetic femoral fractures (PFFs) (3). This is of major importance in the light of the ageing population candidate to THA, which is subsequently leading to an increase in osteoporosis and risk fracture.
Of the various cementing techniques, the French paradox technique has been developed by Marcel Kerboull (Fig. 1) at Cochin Hospital, Paris, France, in the early 1970s. The fundamental principles are based upon the removal of metaphyseal cancellous bone, especially in the calcar region, use of a highly polished and collared filling stem and a simple cementation technique with no pressurization or tip centralizer.
Pr Marcel Kerboull (1934–2020). Image used with permission from the Kerboull family.
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
This technique has demonstrated excellent clinical results and survival, with no long-term stem subsidence (4), and a very low rate of PFF in high-risk patients’ population (5).
History and rationale
Most of the English and American literature recommends a cement thickness around cemented femoral components of at least 2 mm (6, 7, 8, 9, 10). This corresponds to grade A of the classification of Barrack et al. (11), modified by Schmalzried and Harris (12). Indeed, thin areas of cement mantle of less than 2 mm have been associated with fracture of the cement, related to crack propagation by fatigue failure (6, 7, 8, 9, 11). These findings, that have been explained experimentally by high shear stresses and high peak strains at or near the tip of the femoral component (6, 13), have been linked in vivo to an increased risk of debonding and loosening of the cemented femoral component at the stem–cement interface (14). This phenomenon was first reported by Weber and Charnley (15), and was also observed by Marcel Kerboull in 1971 in a series of 320 original flat-back Charnley THAs (DePuy, United Kingdom) performed at Cochin Hospital between 1969 and 1971 (16). At that time, Marcel Kerboull had been using the original Charnley stem for several years. But he had observed that in 8% of the hips, there was a debonding of the stem from the cement associated with a radiolucent line in Gruen et al. zone 1, cement mantle crack at the level of the tip of the stem and sometimes varus tilt of the stem (Fig. 2). This phenomenon, although asymptomatic, was considered as a failure of the fixation. Marcel Kerboull postulated that this thin stem, strongly curved, with a long offset and a relatively closed neck-stem angle put a high pressure on cement in the proximal medial part of the femur. This created high bending stresses at the supero-medial part of the construct, widening the proximal part of the cement mantle and increasing the vertical force on the distal cement, which broke under tensile stress, finally allowing the stem to tilt in varus and subside. He also noticed that the worst case scenario was observed when the femur had a wide canal, and therefore a thick cement mantle. In these situations, the debonding rate was 36% in the hips (Fig. 3A), versus 6% in narrow femurs and 0% in very narrow dysplastic femurs (Fig. 3B). This observation suggested that a stem that fitted the medullary canal close to the cortical bone with a thin cement layer might improve the cemented fixation.
Illustration of the debonding observed by Marcel Kerboull with the firsts Charnley stems implanted in Cochin. The pattern of debonding was a radiolucent line in zone 1 (arrow), with a cement crack at the tip of the stem (arrow) and a varus tilt with a collapse of the supero-medial cancellous bone and cement.
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
In large canal with thick cement, he noticed 36% of debonding (A) vs 6% in narrow canals and 0% in dysplastic very narrow femur (B).
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
From these data, it seemed obvious that the use of an undersized stem to achieve a thick cement mantle was not the good solution to prevent distal migration. This is related to the fact that even a thick cement layer is not able to resist the stresses transmitted by the stem. Marcel Kerboull also understood that cancellous bone, especially in the calcar region that absorbs high stress, was not mechanically resistant enough to carry high load (17) and needed to be removed with hollow reamers or aggressive rasps (Fig. 4). Indeed, under load bearing conditions and ageing, cancellous bone undergoes compression and becomes uneven. In this situation, the cement mantle is subjected to bending and tensile stress and will crack. Thus, removing the cancellous bone gives the cement an even and rigid base, and prevents its crack under bending stress.
Kerboull hollow reamers and aggressive broaches.
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
Marcel Kerboull then modified the stem to design the Charnley-Marcel Kerboull (CMK) stem in order to decrease the level of stress within and at the cement–stem and cement–bone interfaces. The goal was to only subject the cement mantle to compressive stresses and consequently avoid the problems related to the cement layer thickness and resistance.
Three major modifications were made to the original Charnley stem to develop the CMK stem (Fig. 5):
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i) Opening the neck–stem angle from 125° to 130°: to decrease the pressure on the supero-medial and infero-lateral part of the cement mantle.
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ii) Widening the proximal part of the stem: to decrease the stress on the supero-medial and infero-lateral part of the cement mantle through a double taper with a 5° angle. The shear stresses along the stem would be progressively transformed into their compressive component, and the vertical distal force would be dramatically reduced.
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iii) Increasing the range of sizes: to reproduce the patient’s anatomy, have a canal filling and well-aligned stem even in large femurs.
First CMK stem on the left and Charnley stem on the right.
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
Under these conditions, the cement mantle and cement–bone interface were longer subjected to shear stress, and micromotion was reduced to a level tolerated by creep of the cement.
With a canal filling stem, the double tapered shape is acting to decrease the stresses on the cement, whereas with an undersized stem, the distal force will increase and the stem will subside. The CMK is not designed to subside due to the cohesion forces acting on the two polished (stem–cement and cement–bone) surfaces, and micromotion remains below the cement fracture level. The collar may decrease the distal force applied to the cement plug, but does not prevent migration if it occurs. It represents mainly an intraoperative reference to set leg length.
Different surface roughness finishes have been evaluated using similar CMK stem designs. Hamadouche et al. (18) compared the long-term fixation of matte, bead blasted and polished stems (Fig. 6). The survival rate at 13 years, using radiographic loosening as the end point, was 97.3 ± 2.6% for polished, 97.1 ± 2.1% for satin and 78.9 ± 5.8% for matte stems, respectively. Only highly polished stems (Ra = 0.04µ) were retained.
The three types of surface finishes of the CMK stem evaluated in clinical practice.
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
Therefore, the French paradox includes optimal long-term fixation of a cemented stem after removal of the cancellous bone, a thin cement mantle and a highly polished collared stem with no intended subsidence.
(A) Removal of the cancellous bone at the supero-medial region with a curette (here is presented a Hueter anterior approach). (B) Femoral preparation can be performed with either aggressive broaches (no compaction broaches) or reamers. (C) The trial stem has to have rotational stability and seats perfectly on the planned neck cut to restore leg length. (D) Cement is inserted with a syringe or a cement gun after the suction drain has been inserted close to the cement plug. (E) Finally, the stem is inserted and the excess cement is removed with a curette.
Citation: EFORT Open Reviews 10, 6; 10.1530/EOR-2025-0053
Surgical technique
The French paradox technique can be used with any surgical approach. Although initially developed using the trans-trochanteric approach by Kerboull, the evolution of the stem with its shortened version (19) makes it completely possible to use through the anterior Hueter approach (5).
After femoral exposure, the key points of the technique are:
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Femoral neck cut according to the pre-operative planning, as this technique uses a collared stem.
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Removal of the cancellous bone, especially at the supero-medial region with a curette, a hollow or a flexible reamer (Fig. 7A).
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Use of aggressive broaches and/or only hollow reamers until the trial stem or final broach has a good medio-lateral and rotational stability (Fig. 7B). The trial broach or stem needs to have a similar press-fit as required during cementless technique. This simplifies the habits of surgeons familiar with cementless designs (Fig. 7C).
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It is important to note that compaction broaching technique is not part of the French paradox as the aim of this technique is to compact the cancellous bone instead of removing it. Hence, switching intra-operatively to a cemented stem that has the same shape to a cementless straight stem (i.e. Corail-like stem) is not a French paradox technique.
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Plugging and simple washing of the medullary canal.
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Use of standard viscoelasticity cement with antibiotics.
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Simple cement insertion using a syringe or a cement gun (Fig. 7D).
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Pressure is applied by the canal filling stem (Fig. 7E).
In routine practice, after femoral preparation, the femoral canal is thoroughly washed with saline and then dried with a sponge. A suction drain is then inserted close to the cement plug, and standard viscoelasticity cement is inserted with a regular syringe. At this point, if the suction drain brings back blood, this is a sign that it is sucking up the medullary canal in contact with the plug and that the cement is penetrating the entire canal. The highly polished stem is then inserted. The anteversion of the stem has already been chosen with the broaches and the stem is inserted until the collar touches the neck cut. It seems obvious that this technique is much simpler than the very demanding taper slip cementing technique that requires a lot of control for version and depth during stem insertion, along with specific materials that adds to the cost.
In vitro explanation of the French paradox technique
A large body of the literature is available to explain the excellent results of the French paradox in terms of long-term fixation, stability and reduced risk of peri-prosthetic femoral fractures. The principle of the French paradox technique is based upon the metaphyseal cancellous bone removal in order to obtain a canal filling stem; hence, a stable femoral component before cementing. Janssen et al. (20) observed in vitro that canal-filling components produced fewer cement fractures and less rotation than undersized femoral components. Cement mantles surrounded by trabecular bone produced more cement fractures and implant rotation than cement mantles surrounded by cortical bone. Furthermore, Scheerlinck and Casteleyn (21) demonstrated that removing weak cancellous bone favours a direct load transfer to the cortex that is biomechanically stronger bone. Ebramzadeh et al. (17) reported that cancellous bone is not able to carry the load of the stem–cement construct, and that with ageing, this cancellous bone becomes weak and creates a zone of potential failure.
Sevaldsen et al. (22) similarly reported that a cemented Corail component seemed to settle earlier with the line-to-line when compared to standard cementing, and with a lower rate of migration into retroversion. As such, the principle of the French paradox cementing technique could explain a higher torque required to generate a fracture due to increased stiffness of the bone–prosthesis construct. Takegami et al. (23) conducted an in vitro biomechanical study comparing the fracture torque and strain values for the CK, CPT (taper slip) and Versys (collared polished) femoral prostheses. They found a significant difference in fracture torque between the three component types (P = 0.036). The median fracture torque for the CPT femoral component in particular was significantly lower than that for the CK prosthesis (CPT 164.5 Nm versus CK 200.5 Nm; P = 0.046). The strain values for the CPT component were higher than those of the other two implants at the most proximal site. According these authors, and others (23, 24), cobalt–chromium alloy material, polished surface finish, acute-square proximal prosthesis geometry and the absence of a collar may be associated with lower fracture torque, which may be related to increased risk of PFF in some TS designs. Likewise observed with cementless components (25), the collar may have a protective effect with cemented implants. In addition, the TS principle, which allows for the migration of the stem within the cement mantle as a consequence of the viscoelastic properties of bone cement, may increase stress at the bone–cement interface, thereby potentially increasing the risk of fracture.
Clinical results
Only two stems have been validated using the true French paradox principles, the Charnley–Kerboull (16) and the Ceraver Osteal (26), which are both intended to fully occupy the proximal medullary canal of the femur.
Excellent results with stem survivorship higher than 99% at 10 years have been reported with the Ceraver Osteal stem when used with alumina-on-alumina bearings (26, 27) or alumina-on-polyethylene (28). Focal femoral lysis was seen in 3% of cases but only occurred in association with wear and migration of the socket (26). Interestingly, Hernigou et al. (28) reported that the results were less good, with 2% showing aseptic loosening with increased radiolucent lines when there was a poor filling of the diaphysis by the implant. The Ceraver Osteal stem has always been manufactured from anodised Ti6A14V, with a surface roughness of 0.08 μm and a large collar.
By contrast, the Charnley-Kerboull stem has had a number of different versions, all manufactured from stainless steel, and in all of which the collar has been small medially, as with the original Charnley, but larger anteriorly and posteriorly. The best results have been achieved with the polished versions of the stem with a rectangular cross-section (18).
At revision for socket wear and loosening associated with peri-acetabular osteolysis, in the presence of a non-loosened stem, removal of the Charnley-Kerboull stem always revealed an intact and continuous cement mantle. El Masri et al. (4) published the longest report on modern Charnley-Kerboull implants. They evaluated the in vivo migration patterns of 164 primary consecutive Charnley-Kerboull total hip replacements. They used the Ein Bild Roentgen Analyse femoral component (EBRA-FCA) method to assess the subsidence of the femoral component (29). At a mean of 17.3 years (15.1–18.3), the mean subsidence of the entire series was 0.63 mm (0.0–1.94). Nich et al. (30) reported a survival rate of the CK femoral component of 98.6% at 15 years in patient treated for avascular necrosis of the femoral head. The results of the CK stem in younger patients (<50 years) have been reported by Kerboull et al. (31). On their study of 227 hips, they observed a survivorship of 99.4% of polished CK stems after a mean follow-up 14.5 ± 5.1 years.
More recently, a shortened version of the CK stem (AmisK®, Medacta, Switzerland) has been developed after in vitro validation (19) to select an optimal stem length reduction of 12%. This shortened stem has been developed to be more easily introduced through minimally invasive approaches, and to reduce cortical thickening that was observed up to 40% of the hips with the standard length CK implants (4). Laboudie et al. (32) compared the in vivo migration using EBRA-FCA of the 12% shortened implant and the standard length implant with a matched paired cohort of 50 hips in each group. At the 2-year follow-up, the mean subsidence was 0.65 mm (0–1.40) in the AmisK group versus 0.68 mm (0.07–1.43) in the CK group (P = 0.73). When using a 1.5 mm threshold, none of the stems in either group was considered to have subsided. Distal femoral cortical thickening occurred in six of the 50 hips (12%) in the AmisK group versus 20 of the 50 hips (40%) in the CK group (P = 0.003). Laboudie et al. (5) also analysed the rate of PFFs in patients older than 70 years undergoing primary total hip replacement through the Hueter anterior approach. Only the shortened CK stem (AmisK®) was used in their study. Among the 416 THAs, two PFFs (0.48%; 95% confidence interval: 0.13–1.74) were observed, including one Vancouver type B2 fracture 24 days postoperatively and one intraoperative Vancouver type B1 fracture. Valgus malalignment and higher canal bone ratio were found to be associated with PFF. The PFF rate reported in their study was one of the lowest when compared to other cemented stems in the literature, being closer to those of composite beam rather than taper slips designs that shows higher rate of fracture (33, 34, 35, 36).
With the rate of periprosthetic fractures expecting to rise sharply in the near future (37), a great deal of attention is now being paid to the design of cemented and uncemented stems with regards with the risk of PPF. A recent review of 809,832 hips from England’s National Health Data clearly found that only the composite beam technique reduced the rate of PPF compared with cementless collared stems (38). It should be noted that the French paradox was not studied in this review, but as it is closer to the philosophy of composite beam (no subsidence expected), it is likely that the results on a large cohort would be similar. This assumption remains to be demonstrated in large series from registry data.
Conclusions
In conclusion, the French paradox cementation technique is simple, cheap and reproducible. It requires the removal of cancellous bone and the use of a highly polished collared canal filling stem. It has demonstrated excellent long-term results, with no migration expected, and observed up to 18-year follow-up. This technique is associated with an extremely low rate of PPF, including high-risk patients’ populations. Therefore, we are convinced that it represents the method of choice for femoral component fixation, irrespective of the patients’ age or underlying disease.
ICMJE Statement of Interest
MH receives royalties from Medacta, SA, for products related to the topic of this work.
Funding Statement
This work did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sector.
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