Optimal Cementing Technique – The Evidence:

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6.4

Optimal Cementing Technique – The Evidence:

Femoral Pressurisation

Andrew W. McCaskie

Summary

This chapter focuses on cement pressurisation during femoral fixation. It includes theoretical concepts and a description of the supporting evidence in the literature. It then utilises the concepts to describe the surgical technique in a practical »how to do« style. This step-by-step account concentrates on femoral preparation, cement introduction and pressurisation and finally component insertion.

Introduction

Pressurisation is pivotal to optimal cemented femoral fixation. During arthroplasty, a composite structure is created with a central metal prosthesis, an outer cylinder of cortical bone, and between them a layer of bone cement and cancellous bone. This produces two critical interfaces: the cement-bone and prosthesis-cement. The durability of the implant depends on the integrity of the interfaces. These simple facts are clinically important because the operating surgeon is responsible for interface

»manufacture«.

Micro-Interlock

The main technical objective is to drive cement into the trabecular structure of cancellous bone, and create micro-interlock. Any material that occupies the trabeculae during pressurisation will prevent the flow of cement.

This explains why the quality of surface preparation is so important ( chapter 5.1). A further impediment exists, namely the bleeding surface of bone. This is a problem not only in a physical sense but also a dynamic one.

The blood flows in the opposite direction to the desired flow of cement, leading to a potential disruption of the cement-bone interface [3, 18]. The pressure generated by bleeding has been measured at 36 cm of water [15]

which, when the cement is at a low viscosity, could be sufficient to displace cement from trabeculae [16] or cause laminations [5].

Applying pressure to cement can overcome such prob- lems. Firstly, during pressurisation the cement is made to flow in the desired direction, along the pressure gradient.

Secondly, a sustained pressure, above the bleeding pressure, will overcome the effects of bleeding bone.

Pressure, Fluid Flow, and Viscosity

The fluid velocity of an incompressible substance (moving into a porous structure) upon pressure application is de- termined by the Darcy Law [2]. The flow is proportional to the pressure gradient and inversely proportional to the viscosity. In simple terms a greater flow will be achieved as cement viscosity decreases and as the pressure gradient increases. Bone cement changes viscosity throughout its use and the change varies not only with formulation but also with other factors such as the environmental temperature and humidity. This complex relationship has been clarified by research but remains at the heart of the technical challenge faced by the surgeon using cement during surgery.

Understanding the Effects of Pressurisation In terms of cement penetration under applied pressure, Markolf and Amstutz evaluated cement flow through

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holes of differing diameter in an aluminium disc [20].

Penetration depth increased with increasing pore size and applied pressure. Halawa et al. demonstrated the benefit of an applied pressure of 300 KPa, when compared with 150 KPa [13]. Panjabi used a canine model to compare insertion pressures of between 110 and 1230 KPa with a constant pressure of 35 KPa [23]. Relative penetration (percentage of available cancellous bone occupied by cement) increased with insertion pressure. Analysis concluded that 520 KPa was high enough to achieve ad- equate penetration of cement but sufficiently low to avoid complications.

A further question arises over whether an increase in cement penetration produces an improved biomechani- cal performance of the cement-bone interface. Convery and Malcolm reported that 700 KPa achieved 80% more penetration and a 388% increase in shear strength [10].

Askew et al. evaluated the cement penetration and interface strength with different pressures and duration of application [1]. Although both penetration and interface strength increased with increasing pressure, there was no further improvement with longer application times.

From the clinical standpoint, various viscosity options exist. Bean compared standard viscosity cement with low viscosity cement in a human femoral model [4] in which the applied pressure varied. The shear strength increased significantly with pressure until the pressure reached 410 KPa. There was no difference between cements in this respect. A canine model has been used to demon- strate that shear strength at the cement-bone interface is linearly dependent upon depth of cement penetration [17]. An 82% increase in shear strength and 74%

increase in penetration were observed with distal bone plugging and pressurised cement insertion, with lower viscosity cement giving a further increase. In a human femoral model, a retrograde gun technique achieves both improved penetration and interface strength at proximal levels with reduced viscosity cement rather than high viscosity cement [24].

The pressure generated on stem insertion and the flow of cement that results, is another interesting question.

Continuous pressure measurement throughout cementation has demonstrated that stem insertion achieves the highest pressures and it has been suggested that prior impaction of the cement was probably unnecessary [25].

Furthermore, the pressure generated during stem insertion has been measured in a cadaveric femur fitted with pressure transducers [6]. Greater pressures were generated distally (758 KPa for large prostheses and 359 KPa for small) when compared with proximal pressures (200 KPa for large and 131 KPa for small). The timing of prosthesis insertion can affect the pressure generated [9]. Late insertion of a stem creates both increased pressure and intrusion factor compared with early introduction, an effect

enhanced by a tapered design. In addition, a cadaver model has demonstrated less cement-bone radiolucen- cies (significant in zones 2 and 6) with late stem insertion [11].

There are two common ways to generate pressure in clinical practice. First, pressure can be generated before prosthesis insertion using a gun and seal. This utilises a medium or reduced viscosity phase of the cement. This phase is regarded the most crucial and ideally full cement interlock should be achieved at this stage before prosthesis insertion. Second, (distal) pressure can be generated by the prosthesis during insertion. This utilises a relatively higher cement viscosity. An evaluation of pressure generated at all stages has demonstrated the ability of a proximal seal to sustain pressure and also that a component design (Exeter and custom) can generate pressure throughout the length of a cavity model [12].

Clinical Technique and Pressurisation

Over the past 40 years, the debate about cement and how to use it has developed with lack of consensus and varia- tion in practice [14, 21]. Charnley described kneading the cement followed by pressurised insertion with the two-thumb technique [8]. Pressurisation was completed during prosthesis insertion.

The gradual refinement of cementation technique has taken place and there are now higher levels of agreement particularly in terms of gun insertion and pressurisation [22]. The foundation of interlock is the preparation. The following account is clearly a summary of key steps to achieve pressurisation, not a comprehensive account of a modern cementing technique. ⊡Figure 6.8 shows sche- matically how the phases of the technique are related to pressure generation.

Chapter 6.4 · Optimal Cementing Technique – The Evidence: Femoral Pressurisation ¹⁶¹

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0 50 100150 200 250 300

0 2 4 6 8

Time / minutes

Pressure / KPa

Filling

Pressurisation

Prosthesis

Cement

Mixing Mantle

Pressurisation

⊡ Fig. 6.8. This schematic representation shows the phases of femoral cementation in time order and gives an indication of the relative pressures produced. The timings and pressures shown are hypothetical and are for illustration only. They do not represent a timing guide for clinical usage (see text)

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Preparation

▬ Where possible, bleeding from the bone surface should be reduced by control of the blood pressure e.g. hypotensive anaesthesia/epidural.

▬ The production of an even mantle is related to stem position. Therefore, particular care is required to ensure the canal entry point is sufficiently lateral and posterior to minimise both stem malalignment and cement mantle deficiencies ( chapter 5.1).

▬ The reaming and broaching processes should aim to preserve a healthy/sound layer of cancellous bone, with blood supply minimally disrupted. The size of stem and mantle thickness should be planned pre- operatively with templates.

▬ The bone surface should then be cleared of debris by meticulous and copious high-pressure pulsatile lavage (usually 1 liter of irrigation fluid is necessary).

▬ A cement restrictor is inserted 1.5–2 cm distal to the expected tip of the prosthesis. This creates a proximal clean compartment ready for pressure application.

▬ Lavage is repeated and the canal is packed with 3–5%

H₂O₂ or saline soaked ribbon gauze.

Cement Mixing, Introduction and Pressurisation

▬ The author prefers a caulking gun in-syringe mixing system. The technique will require at least 2 mixes (80 g), with more required for capacious canals.

▬ A vent tube placed at the restrictor will remove trapped air and blood as cement is extruded.

▬ Cement insertion begins with canal filling. The modern technique utilises retrograde insertion.

▬ The timing of introduction will depend on the formulation and environment but the author prefers a medium viscosity. It is clearly important to avoid introduction with the cement at such a low viscosity that it cannot be controlled or contained.

▬ Pressurisation (sustained) begins when the canal has been filled. The author’s preferred method is to utilise the cement gun. After retrograde filling the surgeon’s thumb generates pressure, whilst the nozzle is short- ened and a deformable seal positioned around it. The seal is positioned on the femoral neck at the osteotomy site to create a closed compartment ( chapter 2.2).

Slow cement intrusion is achieved by slow steady trig- ger pulls. A sustained pressure can be generated by this technique that encourages cement flow and resists the bleeding pressure. Fat will be seen being displaced from the bone at this point.

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Stem Insertion

▬ The stem is inserted immediately after the release of the seal and pressure, at a time when the cement is at a high viscosity. The timing of insertion will depend on the formulation and environment. It is clearly important to avoid too low a viscosity such that the cement is extruded as the prosthesis is inserted. It is equally important that the viscosity is not so high that the cement cures during insertion before the prosthesis reaches its final position.

▬ The author prefers to maintain a partial seal over the medial neck cut (corresponding to Gruen zone 7) using a thumb, which also guides the implant during insertion. The prosthesis should be inserted with careful regard to position. It is usually possible to use the insertion of the first two thirds of the stem to assess resistance. The final third of stem insertion can therefore be optimised to correspond with maximal working viscosity. No hammering is required.

▬ After reaching the desired position it is important to maintain position until final polymerisation (some surgeons maintain a pressure seal during this period).

Particularly rotation of leg and stem relative to each other should be avoided.

The postoperative radiographic appearance after using this kind of approach is shown in ⊡Fig. 6.9.

⊡ Fig. 6.9. Postoperative radiograph after cemented total hip replace- ment. Note particularly the femoral cement mantle and the optimal cement penetration

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Clinical Outcome with Pressurisation

The change has been supported by clinical evidence. UK joint registers have been used to evaluate patients with loosening at 5 years. Outcome was based on the grade of postoperative cementation derived from radiographs.

Failure was associated with significantly poorer grades of cementation when compared to the non-failure group [7].

A very useful guide to practice is found in the Swedish arthroplasty register [19]. The change to modern cementing technique has been associated with an increase in rates of survival. Moreover, pulsatile lavage, a proximal femoral seal and the use of a distal plug are associated with a reduced risk of revision.

Conclusion

This chapter has reviewed the link between pressure and bone cement, in terms of both theory and practice. Such knowledge when put into practice as part of modern cementation techniques can reduce the risk of revision and increase the survival of the femoral stem. In this way, the surgeon becomes critical in the final »manufacturing«

process of a hip replacement.

Take Home Messages

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▬ Pressure applied to bone cement will direct flow.

▬ Pressure applied to cement, when sustained and of sufficient magnitude can overcome the bleeding pressure.

▬ Flow of cement increases with an increase in applied pressure and a decrease in cement viscosity.

▬ Pressurisation is key to cement intrusion and the creation of micro-interlock.

▬ From a clinical perspective the surgeon makes use of:

– the medium viscosity phase by applying pressure with a sealed sustained technique,

– the high viscosity phase, by applying pressure with prosthesis insertion.

▬ Pressurisation, as a part of modern cementing, has improved the long-term performance of the cemented stem.

References

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