Residual Ridge Resorption: The Unstoppable Phenomenon
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Residual ridge is a portion of residual alveolar bone and its soft tissue covering that remains after the removal of teeth. In the present article, a review on residual ridge resorption is being presented and analysed.

Introduction

After tooth extraction, a cascade of inflammatory reaction is activated and the extraction socket is temporarily closed by a blood clot. Within the first week, epithelial tissue begins its proliferation and migration. Within 2 weeks, active bone formation is seen at the bottom of the socket. In about 6 months, the socket is filled with newly formed bone. Even after the healing of wounds, the residual ridge alveolar bone undergoes a life-long catabolic remodeling. The size of the residual ridge decreases most rapidly in the first 6 months, but the bone resorption activity continues throughout life at a slower rate which results in removal of a large amount of jaw structure. This phenomenon has been described as Residual Ridge Reduction (RRR). Initiation of RRR is always preceded by loss of teeth and of their periodontal membranes, which has the ability to form bone. “Gums have shrunk” is a frequently used expression for RRR.

General Features: RRR is chronic, progressive and irreversible. The rate of resorption is fastest in first 6 months after extraction. Rate is variable between different persons, also within the same person at different times & within the same person at different sites.

Pattern of resorption : The speed and direction of alveolar bone loss is not similar in maxilla and mandible. Mandible resorbs at a faster rate than maxilla. Mandible resorbs downwards & outwards to become wider, while maxilla resorbs upwards & inwards to become smaller. Extensive resorptive changes in one jaw do not necessarily cause resorption in the opposite jaw.

Etiology of RRR: It is postulated that RRR is a multifactorial biomechanical disease that results from a combination of anatomic, metabolic and mechanical determinants. These factors may vary from one patient to the other; different cofactors may combine in an infinite variety of ways, thus explaining the variations in RRR between patients.

  • Anatomic Factors: It is postulated that RRR is dependent on both quantity and quality of the bone of the residual ridges. If there is more amount of bone, then more RRR will be present but the amount of bone is not a good prognostic indicator of the rate of RRR because sometimes large ridges resorb rapidly and some knife-edge ridges may remain with little change for long periods of time. The residual ridges which are high and well-rounded for several years will continue to be so. If it has resorbed in a shorter time, it will resorb at a higher rate. The other factor to be considered is the density of the ridge. But the density at given moment does not signify the current metabolic activity of the bone, and bone can be resorbed by osteoclastic activity regardless of its degree of calcification. Short and square face is generally associated with elevated masticatory forces and a higher rate of resorption.
  • Metabolic Factors: It is postulated that RRR varies directly with certain systemic or localised bone resorptive factors and inversely with certain bone formation factors. RRR is a localised loss of bone on the crest of the residual ridge. It is possible that some of the local biochemical factors that have been studied in relation to periodontal disease could play an important role in RRR. These factors include:
  1. Endotoxins from dental plaque
  2. Osteoclast Activating Factor (OAF)
  3. Prostaglandins
  4. Human gingival bone-resorption stimulating factor
  5. Heparin acts as a cofactor in bone resorption, produced form mast cells found close to the bone margins
  6. Trauma – (especially under ill – fitting dentures), which leads to increased or decreased vascularity and changes in oxygen tension

Systemic factors influence the balance between normal bone formation and bone resorption. Some studies have shown no correlation between the rate of RRR and the presence of osteoporosis whereas there is a hypothesis that osteoporosis could be a contributing factor to the rate of RRR.

  • Mechanical Factors: It is clear that, bone that is “used” by regular physical activity will tend to strengthen within certain limits, while bone that is in “disuse” will tend to atrophy. Some postulate RRR as an inevitable “Disuse atrophy” others postulate that RRR is an “abuse” bone resorption due to excessive forces transmitted through dentures. Both hypotheses sound correct because some patients have little or no RRR and some have severe RRR with or without denture.

Brewer has shown that normal functional masticating and swallowing contacts between dentures average less than 15 minutes per working day. Ohashi et al have demonstrated that swallowing forces averaged 11.4 pounds. Cutright et al calculated that 1500 empty swallows per 24 hours amount to 3500 to 4200 lbs of loading per day. Both positive and negative forces from activities such as smoking, talking, biting should also be considered. Any parafunctional forces from clenching and grinding should also be considered in RRR.

There is tendency for more RRR in mandible than maxilla. Woelfel et al found that maxillary denture area is 4.2 sq inch while the mandibular denture has got 2.3 sq inch. The ratio is 1.8:1. So the amount of bone absorption is more in mandible. The amount of force applied to the bone may be affected inversely by the “damping effect” or energy absorption. This co-factor may be expressed as the “damping effect” may take place in mucoperiosteum, which is considered viscoelastic material. This varies from individual to individual and the absorption quality of the individual may influence the rate of RRR. Frost stated that “bones which are subjected largely to compression load and experience no significant bending loads are composed largely of cancellous bone, which is ideally constructed for the absorption and dissipation of energy. Maxillary ridge is more cancellous than mandible and therefore may be a factor in the differences in the RRR of the jaws.

Gross Pathology of RRR: The primary structural change that occurs in RRR is the loss of bone. The superimposition of portions of tracings of lateral cephalograms with the maxilla and mandible in the following diagram clearly shows the gross reduction of bone in size and shape that occurs on the external surface on the labial, crestal, and lingual aspects of the residual ridge. In order to provide a simplified method for categorizing the most common residual ridge configurations, a system of six orders of residual ridge form has been described by Atwood (1963).

ORDER I – Pre extraction.

ORDER II – Post extraction.

ORDER III – High, Well-rounded

ORDER IV – Knife edge

ORDER V – Low, well rounded

ORDER VI – Depressed.

I II III IV V VI

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Another gross finding is that external cortical surface of the maxilla and mandible are uniformly smooth. The crestal areas of residual ridges show many porosities and imperfections. RRR does not stop with residual ridge but may extend below the apices of the teeth, leaving only a thin cortical plate on the inferior border of the mandible or no alveolar process on the upper jaw. Sometimes a knife edge ridge may be marked by redundant or inflamed soft tissues. Therefore one should always palpate for accurate determination of amount of underlying bone.

Microscopic Pathology: Several microscopic studies have revealed evidence of osteoclastic activity on the external surface of the crest of residual ridges. The scalloped margins of Howship's lacunae sometimes contain visible osteoclasts. There is presence of smooth periosteal lamellar bone on the lingual, inferior and labial surfaces of the mandible, but there is total absence of such lamellations on the crest of the residual ridge. The bony content of the residual ridge is medullary type of trabecular bone. No studies have shown any periosteal lamellar bone over the crest of the residual ridges, but several have shown new bone formation and reversal lines inside the residual ridge. The mucoperiosteum have shown varying degrees of keritinization, acanthosis and edema.

Pathophysiology of RRR: Normally bone undergoes constant remodelling throughout life via bone resorption and bone formation. During growth, bone formation exceeds bone resorption except in osteoporosis, which is a generalised disease of bone in which bone resorption exceeds bone formation. In periodontal disease, there is localized destruction of the bone around teeth, perhaps due to certain local pathologic processes. RRR is a localized pathologic loss of bone which cannot be built back by simply removing the causative factors. If endosteal bone growth does not keep pace with external osteoclastic activity, it would end up with absence of a cortical layer.

Assessment of RRR: Lateral cephalometric radiographs provide the most accurate method for determining the amount and rate of RRR over a period of time. The panoramic radiographic technique described by Wical and Swoope is a simple useful method but provides gross estimate of the amount of RRR. Other methods include absorptiometry, quantitative computed tomography, neutron activation analysis and intraoral microdensitometry.

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Management of RRR – Prosthodontic Principles To Reduce RRR
In order to preserve the alveolar ridge and reduce the amount of stress transferred, certain general principles must be kept in mind during fabrication of complete denture. This may be achieved by

  • Having broad area of coverage under the denture base (to reduce the force per unit area)
  • Use of specialised impression techniques like Mc Cord & Tyson technique, Zafrulla Khan technique, admix technique
  • A decrease in the number of denture teeth
  • Decrease in the buccolingual width of teeth
  • Improved occlusal tooth design form (to decrease the amount of force required to penetrate a bolus of food) by using monoplane teeth
  • During tooth setup the aim should be to reduce the number of inclined planes (to minimise dislodgement of dentures and shear forces) and achieve a centralisation of occlusal contacts (to increase stability of dentures and to maximise compressive load) by using neutrocentric or lingualised occlusal scheme
  • Accurate recording of maxillo-mandibular relationship especially vertical rest dimension which will decrease the frequency and duration of tooth contacts, thereby giving adequate rest to the underlying ridges (Kapur & Soman, 1964; Van Waas, 1990)

Conclusion

Bone resorption of residual ridge is common. The rate of resorption varies among different individuals and within the same individual at different times. Decrease of residual ridges needs to be acknowledged for what it is. It is a major unsolved oral disease which causes physical and psychological problems for millions of people over the world. RRR is a never ending, progressive, permanent and disabling disease of multifactorial origin. At present, the relative importance of various co-factors is not known. It is known about the pathology and the pathophysiology of this oral disease, but we need to know some more about its pathogenesis, epidemiology, and cause. The ultimate goal of research of RRR is to find improved methods of prevention and control of the disease. More research in RRR is required in order to provide the best possible oral healthcare for edentulous patients.

References

  1. Siebert. Some clinical factors related to the rate of resorption of residual ridge. Journal of Prosthetic Dentistry. 1962;12:411-50.
  2. Won Woven. Bone resorption and prosthodontics. Journal of Prosthetic Dentistry. 1973;29:471-13.
  3. Atwood DA. The effect of prosthodontic treatment on alveolar bone loss: A review of the literature.Journal of Prosthetic Dentistry. 1998;80:362-4.

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