DIAGNOSIS AND TREATMENT PLANNING IN FIXED PARTIAL DENTURES
Fixed prosthodontic treatment can offer exceptional satisfaction for both patient and the dentist.
Fixed prosthodontics can transform an unhealthy, unattractive dentition with poor function into a comfortable, healthy occlusion capable of giving years of further service while greatly enhancing esthetics.
To achieve success, requires meticulous attention to every detail from initial patient interview through the active treatment phases to a planned schedule of follow-up care.
Problems encountered during treatment can often be traced to errors
during history taking and initial
Diagnosis: It is the examination of the physical state, evaluation of mental or psychological makeup and understanding the needs of each patient to ensure a predictable result.
Treatment planning: It means developing a course of action that encompasses the ramifications and sequelae of treatment to serve the patientâ€™s needs.
Chief Complaint: It should be recorded in patients own words. The accuracy and significance of patientâ€™s primary reason /reasons should be analyzed first. This will reveal problems and conditions of which the patient is often unaware.
History: A patientâ€™s history should include all necessary information concerning the reasons for seeking treatment, along with any personal details and past medical and dental experiences that are pertinent. A screening questionnaire is useful for history taking. Medical History: An accurate and current general medical history should include any medication the patient is taking as well as all relevant medical conditions. a)
Any disorders that necessitate the use of antibiotic premedication, any use of steroids or anticoagulants and any
previous allergic responses to medication or dental materials should be recorded. b)
Any conditions affecting the treatment plan e.g.: various radiation therapy, haemorrahgic disorders etc. should be recorded.
Possible risk factors to the dentist and auxiliary personnel, e.g. carriers of Hepatitis B, Aids or Syphilis are recorded so that adequate measures can be followed when treating known carriers.
Dental History: Periodontal, restorative and endodontic history are first noted. Orthodontic history should be an integral part of the assessment of a prosthodontic dentition. Occlusal adjustment may be needed to promote long term positional stability of the teeth and reduce or eliminate parafunctional activity. Restorative treatment can often be simplified by minor tooth movement. When a patient is contemplating orthodontic treatment, much time can often be saved if minor tooth movement for restorative reasons is incorporated from the start. TMJ dysfunction history
A history of pain or clicking in the temporomandibular joints or neuromuscular symptoms, such as tenderness to palpation, may be due to TMJ dysfunction which should be treated before fixed prosthodontic treatment begins.
EXTRAORAL EXAMINATION Cervical lymph nodes, TMJ and muscles of mastication are palpated. Temporomandibular joints: The TMJ is palpated bilaterally just anterior to the auricular tragic while having the patient open and close his lower jaw. Tenderness, clicking or pain on movement is noted. Maximum jaw opening less than 40mm indicates jaw restriction, because the average opening is greater than 50mm. Any deviation from the midline is also recorded. Maximum lateral movement can be measured (normal is about 12mm). Muscles of mastication A brief palpation of masseter, temporalis, medial pterygoid, lateral pteregoid, trapezius and sternocleido mastoid muscles may reveal tenderness. The patient may demonstrate limited opening due to spasm of the masseter or temporalis, muscle. Lips: Next, the patient is observed for tooth exposure during normal and exaggerated smiling. This may be critical in treatment planning and particularly for margin placement of metal-ceramic crowns. 5
INTRAORAL EXAMINATION -
First the patientâ€™s general oral hygiene is observed.
The presence or absence of inflammation should be noted along with gingival architecture and stippling. The existence of pockets should be entered in the record and their location and depth chartered.
The presence and amount of tooth mobility should be recorded with special attention paid to any relationship with occlusal prematurities and to potential abutment teeth.
Check for a band of attached gingiva around all the teeth, particularly around teeth to be restored with crowns. Mandibular 3rd molars frequently do not have attached gingiva around the distal segment (30% to 60% of cases).
The presence and location of caries is noted. The amount and location of caries, coupled with an evaluation of plaque retention, can offer some prognosis for new restorations that will be placed. It will also help the preparation designs to be used.
Finally an evaluation should be made of the occlusion. The amount of slide between the retruded position and the position of maximum
interferences if present, should be evaluated. The presence or 6
absence of simultaneous contact on both sides of the mouth should be observed. DIAGNOSTIC CASTS Articulated diagnostic casts are essential in planning fixed prosthodontic treatment. They provide critical information not directly available during the clinical examination, static and dynamic relationships of the teeth can be examined without interference from protective neuromuscular reflexes. They also reveal those aspects of occlusion not detectable within the confines of the mouth. To accomplish their intended goal, they must be accurate reproductions of the maxillary and mandibular arches made from distortion free alginate impressions. (The casts should contain no bubbles as a result of faulty pouring, nor positive nodules on the occlusal surfaces ensuing from air entrapment during the making of the impression). The diagnostic casts should be mounted on a semiadjustable articulator with a face bow. By the use of lateral interocclusal records or check bites, a reasonably accurate simulation of jaw movements will be possible. It is important that the mandibular cast be set in a relationship determined by the patientâ€™s optimum condylar position (centric relation position). 7
Advantages of diagnostic casts: 1)
For diagnosing problems and arriving at a treatment plan.
Allow an unobstructed view of the edentulous spaces and an accurate assessment of the span length, as well as occlusogingival dimension.
Curvature of the arch in the edentulous region can be determined so that it will be possible to predict whether the pontic/pontics will act as a lever arm on the abutment teeth.
Length of the abutment teeth can be accurately gauged to determine which preparation designs will provide adequate retention and resistance.
The true inclination of the abutment teeth will also became evident, so that the problems in a common path of insertion can be anticipated.
Mesiodistal drifting, rotation and faciolingual displacement of prospective abutment teeth can be clearly seen.
A thorough evaluation of wear facets â€“ their number, size and location is possible.
Discrepancies in the occlusal plane become very apparent on the articulated casts.
Occlusal discrepancies can be evaluated and the presence of centric prematurities or excursive interferences can be determined.
10) Teeth that have supraerupted into the opposing edentulous spaces are easily spotted and the amount of correction needed can be determined. 11) Diagnostic wax-up can be carried out in situations calling for the use of pontics which are wider or narrower than the teeth that would normally occupy the edentulous space. Full-mouth Radiographs Radiographs provide the information to help correlate all the facts that have been collected in listening to the patient, examining the mouth and evaluating the diagnostic casts. -
Radiographs should be examined carefully for signs of caries, both on unrestored proximal surfaces and recurring around previous restorations.
The presence of periapical lesions, as well as the existence and quality of previous endodontic treatments, should be noted.
General alveolar bone levels, with particular emphasis on prospective abutment teeth should be observed.
The crown-root ratio of abutment teeth can be calculated. The length, configuration and direction of these roots should also be examined.
Any widening of periodontal ligament should be correlated with occlusal prematurities or occlusal trauma.
Any evaluation of the thickness of cortical plate of bone around the teeth and of the bone trabeculae can be made.
The presence of retained root tips or other pathosis in the edentulous areas should be recorded.
Vitality Testing Prior to any restorative treatment, pulpal health must be assessed, usually by measuring the response to percussion and thermal and electrical stimulation. A diagnosis of non-vitality can be confirmed by preparing a test cavity before the administration of local anesthetic. 10
SELECTION OF AN ARTICULATOR A distinction must be made between mounting for diagnosis and mounting for treatment. The attachment of casts to an articulator for diagnosis will be done with the condyles in a centric relation position. Also when the casts are articulated for restoration of a significant portion of occlusion, it may also be done with condyles in centric relation position. Mounting casts for restoration of only a small part of occlusion will be done with teeth in a portion of maximum intercuspation. Articulators vary widely in the accuracy with which they reproduce the movements of the mandible. 1) At the lower end of scale is a non-adjustable articulator. It is usually a small instrument that is capable of only a hinge opening. The distance between the teeth and the axis of rotation on the small instrument is considerably shorter than in the skull with a resultant loss of accuracy. Drastic differences between the radius of closure on the articulator and in the patientâ€™s mouth can affect the placement of morphologic featuers such as cusps, ridges and grooves on the occlusal surface of the teeth being restored.
2) A semi-adjustable articulator is an instrument whose larger size allows a close approximation of anatomic distance between the axis of rotation and the teeth. If the casts are mounted with a facebow using no more than an approximate transverse horizontal axis, the radius of movement produced on the articulator will reproduce the arc of closure with relative accuracy and any resulting error will be slight. The semiadjustable articulator reproduces the direction and endpoint, but not the intermediate track of some condylar movements. Inter condylar distances are not totally adjustable on semiadjustable articulators. They can be adjusted to small, medium and large configurations. This type of articulator can be used for the fabrication of most single units and fixed partial dentures. 3) A fully adjustable articulator is designed to reproduce the entire character of border movements, including immediate and progressive lateral translation, and the curvature and direction on condylar inclination. Intercondylar distance is completely adjustable. Since this instrument is very expensive and demands high degree of skill and time, it is used primarily for extensive treatment, requiring the reconstruction of an entire occlusion.
(To set the condylar inclinations on a semiadjustable instrument, interocclusal records or check bites are used, when the interocclusal record is removed from an arcon articulator, and the teeth are closed together, the condylar inclination will remain the same. However, when the teeth are closed on a non-arcon articulator, the condylar inclination changes, becoming less steep). Arcon articulators are more widely used because of their accuracy and the ease with which they disassemble to facilitate the occlusal waxing required for cast restorations. This feature makes this type of articulator (arcon) more difficult for arranging denture teeth. The centric position is less easily maintained when occlusion on all of the posterior teeth is being manipulated. Therefore the non-arcon instrument has been more popular for the fabrication of complete dentures. Locating the transverse hinge axis To achieve the highest possible degree of accuracy from an articulator, the casts mounted on it should be closing around an axis of rotation that is as close as possible to the transverse horizontal axis of the patientâ€™s mandible.
A) The most accurate way to determine the hinge axis is by the â€œtrial and errorâ€? method developed by McCollum and Stuart in 1921 (using a kinematic face bow). B) Arbitrary face bows can also used. But they must have an acceptable accuracy. Caliper style ear face bows possess a relatively high degree of accuracy with 75% of the axes located by it falling within 6mm of the true hinge axis. These face bows are designed to be self centering, so that little time is wasted in centering the bite fork and adjusting individual side arms. TREATMENT
RESTORATIONS The most common question arising in treatment planning for single tooth restorations is than in what circumstances should cemented restorations made from cast metal or ceramic be used instead of amalgam or composite resin restorations. The selection of the material and design of the restoration is based on several factors: 1. Destruction of tooth structure: If the amount of destruction previously suffered by the tooth is such that the remaining tooth structure must gain strength and protection from the restoration,
cast metal or ceramic is indicated over amalgam or composite resin. 2. Plaque control: The use of cemented restoration demands the institution and maintenance of good plaque-control program to increase the changes for success of the restoration. Many teeth are seemingly prime candidates for cast metal or ceramic restorations, based solely on amount of tooth destruction that has previously occurred. However, when these teeth are evaluated from the oral environment, they may in fact be poor risks for cemented restorations. 3. Retention: Full veneer crowns are unquestionably the most retentive. However, maximum retention is not nearly as important for single-tooth restorations as it is for fixed partial denture retainers. It does become a special concern for short teeth and removable partial denture abutments. INTRA CORONAL RESTORATIONS When sufficient coronal tooth structure exists to retain and protect a restoration under the anticipated stresses of mastication, an intracoronal restoration can be employed. Here the restoration itself is
dependent on the strength of the remaining tooth structure for structural integrity. a) Glass ionomer: i)
In small lesions where extension can be kept minimal.
Useful for restoring Class 5 lesions caused by erosion or abrasion.
Also employed for incipient lesions on the proximal surfaces of posterior teeth by the use of â€œtunnelâ€? preparation which leaves the marginal ridge intact.
Very useful for the restoration of root caries in geriatric and periodontal patients.
Serves as an interim treatment restoration to assist in the control of a mouth with rampant caries.
b) Composite resin i)
In minor to moderate-lesions in esthetically critical areas.
Due to polymerization shrinkage and insufficient abrasion resistance, its use on posteriors should be restricted to small occlusal and mesio-occlusal restorations on first molars. 16
c) Simple amalgam i)
Simple amalgam, without pins or other auxiliary retention is widely used for one-to-three-surface restoration of minor-tomoderate sized lesions in esthetically non-critical areas.
They are best used when more than half of coronal dentin is intact.
d) Complex amalgam i)
Augmented by pins or other auxiliary means of retention, it can be used to restore teeth with moderate to severe lesions, in which less than half of the coronal dentin remains.
It can be used as a final restoration when a crown is contraindicated because of limited finances. Ideally, however, a crown should be constructed over the pin retained amalgam, using it as a core or foundation restoration.
e) Metal inlay i)
Minor to moderate lesions on teeth where the esthetic requirements are low can be restored with this restoration.
Pre-molars should have one intact marginal ridge to preserve structural integrity.
Additional bulk of the tooth structure found in a molar, permits the use of this type in a MOD configuration.
f) MOD Inlay: i)
Can be used for restoring moderately large lesions on premolars and molars with intact facial and lingual surfaces.
It can accommodate a wide isthmus and up to one missing cusp on a molar.
Cannot be used as a retainer for fixed partial denture.
EXTRA CORONAL RESTORATIONS If insufficient tooth structure exists to retain the restoration within the crown of the tooth, an extracoronal restoration, or crown is needed. a)
Partial veneer crown: i)
Leaves one or more axial surfaces unveneered.
It will provide moderate retention and can be used as a retainer for short span fixed partial dentures.
Full metal crown: i)
To restore teeth with multiple defective axial surfaces or when less than half of coronal dentin remains.
Provides maximum use restricted to situations, where there are no esthetic requirements.
Metal-ceramic crown i)
Provides maximum retention.
Combines full coverage with good cosmetic result.
All-ceramic crown i)
Their use must be restricted to situations likely to produce low to moderate stress usually used for incisors.
Ceramic veneer i)
Produces good cosmetic result on otherwise intact anterior teeth that are marred by severe staining or developmental defects restricted to facial surface of the tooth.
TREATMENT PLANNING FOR REPLACEMENT OF MISSING TEETH Several factors must be weighed when choosing the type of prosthesis to be used in any given situation. Important ones are: a) Biomechanical factors. b) Periodontal factors. c) Esthetics. d) Financial factors. e) Patientâ€™s wishes. Abutment Evaluation -
Abutment teeth are called upon to withstand the forces normally directed to the missing teeth, in addition to those usually applied to the abutments.
Whenever possible an abutment should be a vital tooth. However, a tooth that has been endodontically treated which is asymptomatic with radiographic evidence of a good seal and complete obturation of the canal, can be used as an abutment. If the endodontically treated tooth does not have sound tooth structure, it must treated
through the use of a dowel core, or a pin-retained amalgam or composite resin core. -
Teeth that have been pulp capped in the process of preparing the tooth should not be used as FPD abutments unless they are endodontically treated.
The supporting tissues surrounding the abutment teeth must be healthy and free from inflammation before any prosthesis can be contemplated.
Normally, abutment teeth should not exhibit mobility, since they will be carrying an extra load.
The roots and their supporting tissues should be evaluated for 3 factors: 1. Crown-root ratio. 2. Root configuration. 3. Periodontal ligament area. 1)
Crown root ratio It is a measure of the length of the tooth occlusal to the alveolar
crest of bone compared with the length of the root embedded in the bone. As the level of the alveolar bone moves apically, the lever arm of
that portion out of bone increases and the chance for harmful lateral force is increased. -
The optimum crown-root-ratio for a tooth to be utilized as a fixed partial denture is 2:3 and a 1:1 ratio is the minimum acceptable under normal circumstances.
However, there are situations in which a crown-root-ratio greater than 1:1 (i.e. length of crown greater than length of the tooth) may be considered adequate. If the occlusion opposing a proposed fixed partial denture is comprised of artificial teeth, occlusal force will be diminished, with less stress on abutment teeth. Studies by Klaffenbach in 1936 have shown that occlusal forces exerted against prosthetic appliances has been shown to be considerably less than that against natural teeth. FPD against RPD 26.0lb FPD against FPD 54.4 lb FPD against natural teeth 150.0lb
Root configuration -
Roots that are broader labiolingually are preferable to roots that are round in cross section.
Multirooted posterior teeth with widely separated roots will offer better periodontal support than roots that converge,
configuration. The tooth with conical roots can be used as an abutment for a short span fixed partial denture if all other factors are optimal. -
A single rooted tooth with evidence of irregular configurations or with some curvature in the apical third is preferable to the tooth that has a nearly perfect taper.
Periodontal ligament area: -
Larger teeth have greater surface area and are better able to bear added stress.
Kalkwarf in 1986 showed that millimeter per millimeter, the loss of periodontal support from root resorption is only 1
/3 to Â˝ as critical as the loss of alveolar crestal bone.
Johnston et al in 1971 in their statement designated as â€œAnteâ€™s lawâ€? said that the root surface area of the abutment teeth had to equal or surpass that of the teeth being replaced with pontics.
Fixed partial dentures with short pontic spans have a better prognosis than those with long spans. Failures with long span bridges have been attributed to leverage and torque than overload. Biomechanical factors and material failure play an important role in the failure for long span restorations.
There is evidence that teeth with poor periodontal support can serve successfully as fixed denture abutments in carefully selected cases. Nyman S, Lindhe in 1976 said that teeth with severe bone loss and marked mobility can be used as fixed partial denture and splint abutments. Elimination of mobility is not the goal in such cases, but to prevent further increase in mobility of that tooth. They said that this is possible in highly motivated patients who are proficient in plaque removal.
Biomechanical Considerations All fixed partial dentures, long or short spanned bend and flex. -
Bending or deflection varies directly with the cube of the length and inversely with the cube of occlusogingival thickness of the pontic.
Compared with a fixed partial denture having a single tooth pontic span, a two tooth pontic span will bend 8 times as much. A three tooth pontic will bend 27 times as much as a single pontic.
A pontic with a given occlusogingival dimension will bend 8 times if the pontic thickness is halved. To minimize flexing caused by long/short spans, pontic designs with a greater occlusogingival dimension should be selected. The prosthesis may also be fabricated of an alloy with a higher yield strength, such as nickel-chromium.
The dislodging forces of a fixed partial denture retainer tend to act in a mesiodistal direction, as opposed to the more common buccolingual direction of forces on a single restoration. Preparations should be modified accordingly
to produce greater resistance and structural durability. Multiple grooves, including some on buccal and lingual surfaces are commonly employed for this purpose. -
Double abutments are sometimes used as a means of overcoming problems created by unfavourable crown-root ratios and long span. There are several criteria that must be met, if a secondary abutment is to strengthen the fixed partial denture. a) A secondary abutment must have atleast as much root surface area and as favourable a crown-root ratio as the primary abutment. E.g.: A canine can be used as a secondary abutment to a first premolar primary abutment, but it would be unwise to use a lateral incisor as a secondary abutment to a canine primary abutment.
Arch curvature has its effects on the stresses occurring in a fixed partial denture. When the pontics lie outside the intraabutment axis line, the pontics act as a lever arm which can produce a torquing movement. This is a common problem in replacing all 4 maxillary incisors with 26
a fixed partial denture. The best way to offset this torque is by gaining additional retention in the opposite direction of the lever arm. The secondary retention must be at a distance equal to the length of the lever arm from the interabutment axis. -
E.g.: The first pre-molars some times are used as secondary abutments for maxillary four-pontic canine-tocanine FPD.
SPECIAL PROBLEMS A)
Pier abutments: An edentulous space can occur on both sides of a tooth, creating a lone, freestanding pier abutment. Physiologic tooth movement, arch position of the abutments and a disparity in the retentive capacity of the retainers can make a rigid 5-unit fixed partial denture as a less than ideal plan of treatment. -
It has been theorized that forces are transmitted to the terminal retainers as a result of the middle abutment acting as a fulcrum, causing failure of the weaker retainer. However a photoelastic stress analysis study conducted by
Standlee and Caputo in 1988 has shown that the prosthesis bends rather than rocking. -
The retention on the smaller anterior tooth is usually less than that of the posterior tooth because of its smaller dimensions. The loosened casting will leak around the margin and caries is likely to become extensive before discovery.
The use of a non-rigid connector has been recommended to reduce this hazard. The movement in a non-rigid connector is enough to prevent the transfer of stresses from the segment being loaded to the rest of the FPD.
The most commonly used non-rigid design is a T shaped key that is attached to the pontic and a dove tail key way placed within a retainer.
The key way of the connector should be placed within the normal distal contours of the pier abutment and the key should be placed on the mesial side of the distal pontic.
B) Tilted Molar Abutments
A common problem that occurs is the mandibular second molar abutment that has tilted mesially into the space formerly occupied by the first molar. There is further complication if 3rd molar is present. It will usually have drifted and tilted with the 2nd molar. -
If the encroachment is slight, the problem can be remedied by restoring or recontouring the mesial surface of the third molar with an overtapered preparation on the second molar.
If the tilting is severe, other corrective measure will have to be followed. The treatment of choice is uprighting of the molar by orthodontic treatment. The third molar if present is often removed to facilitate the distal movement of the 2nd molar. After removal of the appliance, the teeth are prepared and a temporary FPD is fabricated to prevent post treatment relapse.
A proximal half crown can be used as a retainer on the distal abutment. This preparation design is a 3 Âź crown that has been rotated 90Â°. It can be used only if the distal surface is untouched by caries.
A telescoping crown and coping can also be used as a retainer for the tilted molar. A full crown preparation with heavy reduction is made to follow the long axis of the tilted molar. An inner coping is made to fit the tooth preparation. The proximal half crown that will serve as the retainer for the FPD is fitted over the coping.
A non-rigid connector is another solution to the problem. A full crown preparation is done on the tilted molar, with its path of insertion parallel with the long axis. A box form is placed on the distal surface of the premolar to accommodate a keyway in the distal of the premolar crown.
C) Canine Replacement Fpds This is a problem because often the canine lies outside the interabutment axis. The abutments are the lateral incisor, usually the weakest in the entire arch and the first premolar, the weakest posterior tooth. A FPD replacing maxillary canine is subjected to more stress than that replacing a mandibular canine, since forces are transmitted outward on the maxillary arch. So the support from secondary abutments will have to be considered. An edentulous space created by
the loss of a canine and any 2 contiguous teeth is better restored with a removable partial denture. D) Cantilever FPDs A cantilever FPD is one that has an abutment or abutments at one end only, with the other end of the pontic remaining unattached. This is a potentially destructive design with the lever arm created by the pontic. -
Abutment teeth for cantilever FPDs should be evaluated for lengthy roots with a favourable configuration, good crown root ratios and long clinical crowns.
Generally, cantilever FPDs should replace only one tooth and have atleast 2 abutments.
A cantilever can be used for replacing a maxillary lateral incisor with canine as the abutment. There should be no occlusal contact on the pontic in either centric or lateral excursions.
A cantilever pontic can also be used to replace a missing 1st premolar with second premolar and 1st molar as abutment. The occlusal contact should be limited to the distal fossa on the 1 st premolar pontic.
Cantilever FPDs can also be used to replace molars when there is no distal abutment present. Most commonly the 1 st molar is replaced with the 2 premolars as abutments. The pontic should have maximum occlusogingival height, there should be light occlusal contact on the pontic with no contact in any excursions. Buccolingual width should be kept minimum and the pontic should resemble more of a premolar.
Conclusion The scope of fixed prosthodontic treatment can range from the restoration of a single tooth to the rehabilitation of the entire occlusion. Single teeth can be restored to full function and improvement in cosmetic effect can be achieved. Missing teeth can be replaced with fixed prosthesis that will improve patient comfort and masticatory ability, maintain the health and integrity of the dental arches, in many instances elevate the patientâ€™s self image. It is also possible by the use of fixed restorations, to render supportive and long range corrective measures for the treatment of problems related to the temporomandibular joint and its neuromuscular system. On the other hand, with improper treatment of the occlusion it is possible to create disharmony and damage to the stomatognathic system. Bibliography 1)
Kalkwarf K.L., Krejci R.F., Pao Y.C. : Effect of root resorption on periodontal support. J.P.D. 1986; 56: 317-319.
Malone W.F.P., Koth D.L., Cavazos E. : Tylmanâ€™s theory of practice of fixed prosthodontics. 8th Ed., Ishiyaku publications, 1977.
Markley M.R. : Broken-stress principle and design in fixed prosthesis. J.P.D., 1951; 1: 416-423.
prosthodontics. J.P.D., 1968; 19: 483-488. 5)
Rosenstiel R.F., Land M.F., Fujimoto J. : Contemporary fixed prosthodontics. 1st Ed., Mosby Publications, 1988.
Shillingburg H.T., Hobo S., Whisett L.D., Jacobi R., Brackett S.E. : Fundamentals of fixed prosthodontics, 3rd Ed., Quintessence Publication, 1997.
Sutherland J.K., Holland G.A. : A photoelastic analysis of the stress distribution in bone supporting fixed partial denture of rigid and non-rigid designs. J.P.D., 1980; 44: 616-23.