Discuss_the_Strategies_for_Dealing_with_Poor_Bone_Quality

Discuss the strategies for dealing with poor bone quality Nilesh R Shah Introduction The process of osseointegration has been documented since the sixties. Implant dentistry has progressed dramatically since then. With the mean age of the population increasing, demand for implant rehabilitation is rising. Implants are being used increasingly to restore function in areas of poor bone quality. Many methods have been developed to increase success rates in these areas. The use of longer wider implants which have a roughened surface show encouraging success rates in poor bone quality. But there are very few long term studies about the effects of the surface topography. There is also a tendency for health professionals to rely ‘on new science’ to ensure success, it is equally important that the old proven protocols are not readily dismissed In favour of new unfounded ones. This essay will look critically at the various methods to maximise implant success and how these methods have been extrapolated to treat areas of poor bone quality. What is poor bone quality' The classification of bone quality and quantity was described in 1985 by Lekholm and Zarb (Lekholm and Zarb, 1985)The classification was based on their own clinical studies. They proposed a differentiation of jawbone quantity (A to E), and jawbone quality (1 to 4) in the anterior regions of the jaw. Quantity (shape): A Unresorbed alveolar bone B Some resorption of alveolar bone C Complete resorption of alveolar bone D Some resorption of basal bone E Extreme resorption of basal bone Quality 1 Primarily cortical bone 2 Thick cortex with dense cancellous bone 3 Thin cortex with dense cancellous bone 4 Thin cortex with low density cancellous bone [pic] Fig 1 Quantity & quality according to Lekholm and Zarb (Lekholm and Zarb, 1985) The classification was arrived at using radiographs, without any method of standardised assessment. Back to basics – Treatment Planning & Surgical techniques Systematic and considered treatment planning is essential to any dental procedure to maximise success. Ill conceived and hurried treatment planning often leads to poor, compromised and even failed results(Jaffin and Berman, 1991) . The treatment planning experience and surgical skills of an operator have a direct impact on treatment outcome. Lambert et al(Lambert et al., 1997) looked the impact of the learning curve for implant placement. They found that implants placed by inexperienced surgeons failed twice as often as those placed by experienced surgeons. In type 3 and 4 bone, surgical skill and technique is of greater importance than in type 1 and 2 bone. Many differing surgical techniques have been proposed to help increase the success rates of implants in the posterior maxilla , which normally consists of type 3 or 4 bone. Site preparation The use of osteotomes or bone condensers – they work on the principal that bone is not removed from osteotomy site but compacted, this enhances the density around the implant. The technique described by Summers(Summers, 1994;Summers, 1996;Davarpanah et al., 2001) has proved very useful to me in areas of poor quality bone. Not only do you benefit by compacting the bone, but the proprioceptive feedback I get when using the technique enables me to optimise the implant angulation and depth of the osteotomy. It is now my instrument of choice when placing implants in type 3 and 4 bone. Modification of drilling techniques – copious amounts of irrigation is required when drilling bone. Many systems have an internal irrigation which enables the coolant to be delivered efficiently at the cutting tip where most heat is generated. In poor quality bone, it is recommended that the final drill used is smaller in diameter than the implant, and that it is only used to drilled 75% of the final fixture length(Bahat, 2000) Anatomical considerations Bicortical fixation – Branemark(Branemark et al., 1984) reported a 10% higher failure rate for maxillary implants that perforated the floor of the maxillary sinuses. Looking at animal studies, Ivanoff (Ivanoff et al., 1996) found that bicortical anchorage greatly enhanced the removal torque values when compared to monocortical fixation. Use of alternate anatomical sites: Pterygoid plate (Graves, 1994) – only avoids sites of poor bone quality Zygomatic implants – developed by Nobel Biocare can provide excellent posterior maxillary anchorage.(Bedrossian et al., 2002) Implant variables Angulation – no differences in failure rate is seen between straight and angled abutments(Sethi et al., 2000) Number– Bahat (Bahat, 2000) had a 95% success rate over 5 years in the posterior maxilla. He recommends one implant per unit to be restored. Length – the longer the implant the larger the surface area. Jaffin (Jaffin and Berman, 1991) found that longer implants had a better prognosis than shorter implants in type 4 bone. Width - the wide fixture could help in type 4 bone by effectively engaging the denser cortical plates laterally. With increasing width, the surface area of bone to implant contact is increased. If bone to implant contact is a measure of implant osseointegration then this in theory should allow fixtures to be placed which are shorter and in poor bone should increase the surface area to which osteoblasts can adhere to. Shorter wider fixtures may have similar surface areas to longer regular implants, but there biomechanical response to loading may be completely different. It is probably better to have a longer implant as the distal implant in a cantilever scenario compared to a wide short implant. The longer implant would probably resist the tension and compression forces better than the sorter implant. Van Steenberghe et al(van Steenberghe et al., 1990) used 4.0mm fixtures in types 3 and 4 bone and found that there were fewer failures when compared to standard 3.75mm fixtures. Posterior teeth having 2-3 roots have a total root surface area of 450 to 533mm2. A 3.75 mm implant has a surface area of 72 to 256mm2 depending on its length. (Bahat and Handelsman, 1996) As can be seen from figure 2, wider fixtures have a much greater surface area than smaller diameter fixtures. Fig 2. Surface area (mm2) of 10mm long Nobelpharma implants [pic] Bahat(Bahat and Handelsman, 1996) compared a single 5mm or 5mm+ wide fixtures to two 3.75mm or 4mm fixtures replacing a single molar tooth. He found that both techniques very successful, overall failure rates were 2.3% for wide fixtures and 1.6% for double implants. It was noted that both techniques were greatly dependent on the skill of the operator, leaving very little margin for errors during surgery. Fig. 3 Comparison of wide implants with the use of 2 regular implants to replace a single molar tooth |Wide implants |2 Implants | |Easier to place surgically |Mimics multi rooted teeth | |Technique sensitive |Technically difficult | | |More space required | | |Difficult to restore | | |Harder for patient to clean | | |Rotational forces are better resisted | Other surgical considerations Operating conditions - A study by Scarf and Tarnow (Scharf and Tarnow, 1993a) looked at implant success rate under ‘clean’ versus sterile conditions. The results indicated that implant surgery can be performed under both "sterile" and "clean" conditions to achieve the same high rate of clinical osseointegration. Although there are no studies to say that sterile conditions are a prerequisite for implant surgery, most dentists would adhere to strict aseptic protocol for medico legal issues at the very least. Incision technique has also been studied, the type of incisions seems to have no significant bearing on implant success rates(Scharf and Tarnow, 1993b). I have found that remote incisions are better. If the soft tissue becomes infected or wound dehiscence occurs, the implant site being remote to the primary site is not readily compromised. It does make the surgery more difficult, and is not really an issue if healing is uneventful. Primary stability Primary stability a measure of the resistance to movement when an implant is first placed into bone. It seems to be a critical factor in initial implant success and early osseointegration. The original Branemark protocol (Branemark et al., 1985)called for implants to be buried and unloaded in the mandible for 3 months and the maxilla for 6 months due to poor quality bone. These initial periods were developed from data using animal studies, and are now being challenged due to a number of factors: Advances in understanding the process of osseointegration • Changes of implant design – Macro: Shape Surface Material Micro: Surface treatments Bioactive materials • Commercial pressures – the number of implants companies has increased 10 fold in the last decade, the majority claiming shorter healing and loading times. The claims are often based on poor long-term research or no research at all. • Patient driven – the demands to provide long lasting high quality dentistry quick, is a real problem for many dentists. Patients are able to source enormous quantities of information on the internet, which often make wild claims and raise the expectations of the patient. Although the tendency is to push the boundaries of established protocols, one has to look at the best available evidence and make a judgment if protocols should be challenged. Brunski (Brunski et al., 1979) was one of the first people to describe a fibrous tissue interposition at the bone implant interface due to micromotion when implants were immediately placed into function. There have been numerous studies that show similar outcomes. Nowadays the concept excessive micromotion is emerging. The concept of threshold micro movement was first suggested by Cameron (Cameron et al., 1973)His team while looking at bone healing noticed that not all movement led to a fibrous union. Two different types of movement were recognized micromovement and macromovements. On one hand micromovement did not prevent bone ingrowth into porous implants; on the other hand macromovements did prevent the ingrowth of bone and resulted in fibrous tissue interposition. Brunski (Brunski, 1993)proposed a rule of thumb for the effect of micromotion on a bioinert implant: Below 50 um - boney healing Above 150um – fibrous healing The proviso was made that if the macro or microdesign of implant was altered then the values may differ. Once an implant is placed, its ability to resist micromovement is dependent on how it to dissipates forces to the surrounding bone. In dense bone, such as that found in the anterior mandible, this is rarely a problem even if the implants are loaded immediately. In poor quality bone, the cortical plate is often thin, trabecular bone often contains a high percentage of soft marrow and fat which cannot efficiently resist these forces, resulting in micromovement above a physiologically level and poor primary stability. Implant design Optimal Implant design should take into consideration a number of factors(Kohn, 1992) [pic] Fig. 4 Schematic of independent engineering factors affecting the success of dental implants Macro design considerations Material - most implants are made out of commercially pure titanium. It has excellent corrosion resistance, high strength and is biocompatible. It is how ever not osteoinductive (Davies, 1996;Davies, 1998) and does not induce the formation of new bone. Threads – they improve primary stability, increase surface area and reduce microstrain at the bone implant interface. Nobel Biocare developed a fixture specifically for low density bone. It has a double thread pattern which generates less heat on insertion, and increases the bone to implant contact. Shape – implants have evolved from flat blade type designs to cylinders. Essentially all the manufactures have kept to the cylinder design with slight variations. Root form implants are popular for immediate implants. Micro design considerations Currently one of the most researched design features is the surface topography of dental implants. For the purposes of this essay, machined implants shall be taken as having a smooth surface. A Change in surface topography from smooth to rough has the effect of , increasing the surface area. The size and orientation of various roughening techniques seems also to influence cells in differing ways(Chehroudi et al., 1990). Many surface treatments in combination with differing coatings are available: 1. Machined 2. Titanium Plasma Sprayed (TPS) – can be coated with hydroxyapeptite (HA) 3. Acid Etched – using hydrochloric acid and sulphuric acid 4. Sandblasted – various grit sizes can be used 5. Sintering – molten spherical powders are sintered on to the implant surface 6. Combination – E.g. SLA commercially used by Straumann. The surface is first sandblasted with large grit sand, and then acid etched. This then produces a semi-porous microstructure. Various methods have been devised on how to test the healing capabilities of implants: 1. Pull out test - the implant is pulled out of bone by force in the long axis of the implant 2. Push out test – the implant is pushed out of bone by force in the long axis of the implant 3. Torque – most commonly used, the rotational force to remove the implant is measured 4. Histology – a histological specimen is prepared, and the amount of bone contact on the implant surface is measured over a representative sample and expressed as a percentage of Bone-implant contact (BIC) 5. Clinical observations – looking at longevity and bone loss primarily None of these tests are ideal. Many studies have there own ‘slant’ on the tests making it almost impossible to make valid comparisons between the various surfaces. Bauser (Buser et al., 1999) looked at 3 surfaces TPS, SLA and machined in miniature pigs. All test Implants had the same macro design n and dimensions. He found that the removal torque values (RTV) for the SLA and TPS implants in areas of poor bone quality was significantly higher than that of machine implants after 8 and 12 weeks of healing. [pic] Fig.5 RTV’s in different anatomical positions(Buser et al., 1999) Many other studies to demonstrate that a roughed surface significantly improves the bone implant contact compared to machined implant. This seems to be more important during the early stages of healing compared to implants that are established. It is now generally accepted that in areas of poor bone quality, a roughed implant is preferred to machined. The future of surface could lie with the impregnation of the surfaces with Bone Morphogenic Proteins (BMP’s). They have already been used successfully in orthopaedics(Oreffo and Triffitt, 1999), but the cost of manufacture may prove prohibitive for wide spread use in dentistry. Other important factors Prosthetic design – thought at the treatment planning stage should be given about the prosthetic requirements for the case. If treatment planned well this stage should be straight forward. There is no evidence to say that removable prosthesis are better than fixed counterparts. Each case has to be viewed and planned individually to ensure success. Well fitting and correctly constructed superstructures are essential for longevity. Conclusions Poor quality bone has been effectively classified(Lekholm and Zarb, 1985) .We have the means of identifying these areas at the treatment planning stage. It is essential that the dentist plans every stage meticulously, from the final restoration to the surgical technique. Only then will he ensure repeated success in difficult areas. Many new claims are made, almost on a daily basis from implant manufacturers. They are commercially driven and will claim that their product is superior. 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