Dimensional changes in casting denture frameworks
titanium
Ronald Blackman, D.D.S., M.S.D.,* Nasser Barghi, D.D.S., M.A.,** Christopher Tran*** University of ‘Texas Health Science Center, Dental School, San Antonio, Texas
removable
partial
and
The difficulties encountered in casting titanium and its alloys have until recently hindered any widespread use of titanium in restorative dentistry. Now both equipment and materials are available for the routine use of titanium in the dental laboratory. This study used one of the casting systems and examined dimensional changes tha.t occur during the construction of a removable partial denture framework. Nineteen castings were measured for horizontal and vertical plane changes. Results show these castings to have cross arch contraction, vertical plane expansion, and near neutral anterioposterior change. It seems that pure titanium is within the range of dimension changes generally accepted for base metal removable partial denture alloys of the nickel-chromium variety. (J PROSTEET DENT 1991;65:309-15.)
A
lthough titanium is not new to dentistry, its application in making cast restorations introduces a new class of metals for dental treatments. Titanium casting technology has been developed and refined over many years in the aerospace industry, where casting to near net shape results in great labor and material savings. From an industrial viewpoint, titanium is attractive for its low weight-to-volume, high strength-to-weight, fatigue resistance, and corrosion resistance.’ These characteristics are desirable in dentistry, but interest is more sharply focused on biocompatibility. Titanium is hypoallergenic and possessesmany of the clinically favored properties of type III and IV dental gold alloys2 Dental interest in titanium has been restrained by the inability to overcome inherent casting problems. Now that titanium casting technology is reasonably economical and practical, its application for routine castings can be examined. Titanium casting requirements deviate considerably from those of common dental alloys. The melting point of pure titanium is 1720’ C (3646’ F) and is usually achieved with an electric arc melting method not used in dentistry. Molten titanium iis extremely reactive with other elements such as nitrogen and oxygen and with compounds such as the silica used in casting investments. When cooling from a molten state, titanium crystalizes in an alpha phase below 883’ C (1872” F). Alpha phase mechanical properties are similar to those of type III and IV dental gold alloys. Above the critical 883’ C temperature, crystalization
*AssociateProfessor,Department of Restorative Dentistry. **Professor and Head, Division of Occlusion, Department of Restorative Dentistry. ***Second year dental student. 10/I/24205
TEE JOURNAL
OF PROSTHETIC
DENTISTRY
occurs in a beta phase characterized by brittleness and increased strength. For this reason, temperature control after casting is important and must be included in a complete casting process. Titanium’s light weight presents another formidable obstacle for common centrifugal force casting methods. Its atomic weight is 47.90, making it one half as heavy as typical Ni-Cr alloys and one fourth as heavy aa high gold alloys. Therefore new and expensive casting machines are necessary. Two systems available for dental use have resolved these problems differently. One (Ohara Company, Osaka, Japan) uses centrifugal force generated by a powerful motorwound spring and an argon gas melting and casting environment. The other (Iwatani Corp., Osaka, Japan) uses a vacuum/pressure casting machine with electric arc melting, in an argon gas environment. Also, industrial progress has been made with titanium alloys. A 96% alloy, titanium, aluminum, and vanadium (Ti-6Al-4V), has been highly developed, particularly with post-casting processes for improving physical properties.3 Both this alloy and pure titanium are currently used for dental endosaeous implants4 Recent studies have examined a wide variety of titanium alloys that may prove to be more ideal for dental castings than either pure titanium or Ti-6Al-4V, however, they are not yet offered commercially for dental use.5-7The application of titanium to dentistry’s popular metal-ceramic technology has been successfully investigated* and awaits commercial development. Titanium’s high melting temperature and low specific gravity should provide exceptional sag resistance during the porcelain sintering process. Some comparisons of titanium with other removable partial denture (RPD) casting metals, derived from various sources, are given in Table I. It can be seen that pure titanium RPD castings have properties similar to long-favored type IV gold alloys. This study measured dimensional changes in large dental castings made using the equipment,
309
BLACKMAN,
Vertical Plane Reference Points
BARGHI,
AND
TRAN
Refractory Cast and Investment Mold Relations
d Vertical plane dimensions [ approx.] ocdusal to palate 16mm 6mm 1omnl
Fig. 1. Cast reference point locations for vertical plane measurements. These marks transferred from master metal die were all in the same coronal plane. Palatal plane and ridge plane were parallel to each-other. -
Horizontal Reference Points and Dimensions Horizontal plane dimensions [approx]: 4Smm AbD 5omm AlDE 41 mm AbX 39mm BbC DtoE 53mm 13mm AbBC BCbDE 2Smm
Fig. 2. Reference point locations in horizontal plane used for measurements. Approximate distances between points and calculated distance AX are indicated.
materials, and procedures of the Ohara Company. RPD master cast measurements were compared with those of titanium castings made for the same casts. Differences reflect composite dimensional changes between the RPD castings and their individual master casts. Similar studies using a popular nickel-chromium (Ni-Cr) alloy (Ticonium 100, Ticonium Company, Albany, N.Y.) provide comparisons for these titanium castings.g9lo
METHODS
AND MATERIAL
Twenty titanium, 99.5% commercially pure (CP), RPD castings were made using Ohara Company equipment with the recommended materials. Technical aspects of this system have been described in detail by Szurgot et al.lr and the procedures for this project followed their outline closely with only minor exceptions in duplicating and furnace processing temperatures. The RPD master casts were 20
310
\,
/~TgzJii~ [ minor sprues and vents deleted ]
Fig. 3. Cast and mold relations. Vertical counterclockwise spin of centrifugal casting machine placed anterior segment of casting in trailing position. Sprue curvature positioned occlusal plane perpendicular to rotating arm of casting machine.
Cast RPD Frame As Recovered E
Fig. 4. Appearance of recovered RPD framework casting. Preparation for measurements included cutting off stainless steel rods on underside of casting flush with surface, and finishing rod ends with fine abrasives.
dental stone duplicates of a master metal die stylized in the shape and size of a human maxilla (Fig. 1). The die had measurement reference marks in the forms of Xs cut in the palatal vault and edentulous ridges. Reference marks for the accusal plane consisted of five small holes (0.84 mm in diameter and 4 mm long) placed with a drill press in the duplicated stone master casts (Fig. 2). The holes were perpendicular to the occlusal plane and were located in positions representative of molars, premolars, and a central incisor. A stainless steel rod 0.76 mm in diameter and 8 mm long was placed in each hole prior to duplicating the cast with reversible hydrocolloid. The rods were transferred to the refractory cast by the duplicating material in the same alignment and position. After removal from the duplicat-
FEBRUARY
1991
VOLUME
65
NUMBER
2
CASTING
TITANIUM
REMOVABLE
PARTIAL
DENTURES
Table I. Cast titanium and RPD alloys Casting
Metal Ti, 99.5% CP
Ti, 99.5% CP Ti, 99.5% CP Co-Cr Ni-Cr Au, type IV
machine
Electric arc, centrifugal Electric arc vacuum/pressure Electric arc, centrifugal Induction, centrifugal Induction, centrifugal Gas/air torch, centrifugal
CP, Commercially pure; MOE, measure of effectiveness; ber KHN, Knoop hardness number.
MOE (GN/m2)
96 228 186 90 UTS, ultimate
UTS (MPa)
735 415 540 640 800 770
JOURNAL
OF :PROSTRETIC
DENTISTRY
% Elongation
336 383 495 690 495
18.0
7.9 7.9 1.5 1.7 6.0
Sp.gr. (gm/cm3)
4.5 4.5 4.5 8.3 7.5 15.2
Melting Temp. (“C)
Hardness
215 VHN 191 KHN 236 KHN 380 VHN 340 VHN 235 VHN
1720 1700 1700 1450 1275 950
tensile strength; YS, yield strength; Sp.gr., specific gravity; VHN, Vickers hardness num-
ing material, the refractory casts with the rods in place were oven-dried and sealed with a resin spray. All 20 casts were waxed for RPD frameworks in the same manner, beginning with 24-gauge (approximately 0.5 mm) adhesive sheet wax cut to follow the form of a template. Minor connectors were made using a double thickness (approximately 1 mm) of the same sheet wax. Frame design and major sprue configurations are illustrated in Fig. 3. A minor sprue was attached to each rest, which contained a stainless steel rod. The mold crucible was developed with a plastic form and was connected to the pattern by wax sprues (6-gauge major sprues and lo-gauge minor sprues). Waxed and sprued refractory casts were invested to form ringless molds 7.8 cm long x 8.6 cm in diameter. Fifteen 18-gauge wax per:ipheral vents were joined into groups on the underside of the refractory cast before investing. They formed three open channels through the mold to the rear surface. Wax was eliminated and molds were heat-soaked in a conventional burnout furnace. These refractory molds were then transferred to a high-temperature processing furnace for mold expansion. The temperature schedule is outlined in Table II. After heat processing, refractory molds were allowed to cool in the furnace until they could be picked up with unprotected hands and placed in the casting machine. Each casting was made with a 40 gm pure titanium ingot and a machine setting of 180 A, 38 windings, and 30 psi argon gas.Recovered castings were air abraded with sand and 25 pm aluminum oxide. Sprues and vents were removed (Fig. 4). Reference rods on the underside of rest seats were cut flush with the surface, finished with fine abrasive points, and air abraded with aluminum oxide. Of the 20 castings, one was eliminated as defective and one other had minor defects that did not interfere with its use for measurements. Nineteen RPD frameworks and their individual stone master casts were measured in both horizontal and vertical planes and were compared for dimensional changes. Casts and castings were stabilized and leveled in a vise and horizontal measurements were made with a microscope at 50 power magnification (Gaertner Model M1142, Gaertner Scientific Co.rp., Chicago, Ill). Distances AD, AE, BC,
TEE
(::a)
Table II.
Casting mold preparation Phase
Burn-out 20 to 800” c Hold temp. Processing cycle 800 to 950’ C Hold temp. 950 to 1200° c Hold temp. Cool to 40” c
Time
60 minutes 30 minutes 15 minutes 15 minutes 30 minutes 30 minutes 75 minutes
and DE (Fig. 2) were measured directly. AX, the perpendicular distance between line DE and point A, was calculated. The location of points used in these measurements are identified in Figs. 1 and 2. The stone master casts and RPD frameworks were similarly mounted and leveled for the Z axis (vertical plane) measurements. These measurements were made between the palatal vault and the plane defined by the two edentulous ridges. Vertical measurements were made directly with a digital linear dial gauge (Model EG-100, Ono Sokki, Japan) at positions designated in Figs. 1 and 5, adjacent to reference marks. All were in the same cross-sectional plane, transferred from the metal die onto the master casts and titanium frameworks. These marks were located on RPD framework elements representing a palatal strap and edentulous ridge bases. To obtain single vertical measurements, distances at points a and d, and at b and c were averaged for the calculations. Data were analyzed to determine whether and to what degree a relationship existed between the identified variables using the Pearson r and Spearman rho correlation coefficients.
RESULTS Table III shows horizontal and vertical plane measurement differences, in percentage form, for the 19 castings compared. A relatively wide spread was noticeable in the measurements from casting to casting, particularly in the
311
BLACKMAN,
Vertical Plane Frame Reference Point Positions i---, . -
Horizontal
Shrinkage
AND TRAN
Pattern Master cast size comparison
A
I
BARGAI,
I
AX increase [small diflerence between an@ DAE and DE]
Ax decrease (larae dll%r%nce belween angle DAE and DE)
Fig. 6. Illustration showing location of reference points on underside (tissue side) of center section of RPD castings. Measurements in vertical plane were obtained using points immediately medial to these marks.
distance DE (Figs. 4 and 5). Mean differences were: AD, -0656%;AE, -0+704%;BC,-0.997%;andDE,-2.564%. Results are reported in percentage form to facilitate comparisons. The calculated distance AX with a mean difference of 0.092% showed considerable dependence on the distance DE and the angle DAE, both elements of triangle DAE. When the percent shrinkage of DE was proportionately close to the percent shrinkage of the angle DAE, AX showed expansion rather than shrinkage even though measured distances all indicated shrinkage (Fig. 6). The parallel cross arch measurements BC and DE had disproportionate shrinkages, such that DE shrinkages were much greater than length differences between BC and DE would suggest. To illustrate this, lines joining the castings’ horizontal reference points would be curved and distorted representations of AD and AE would occur. In the vertical plane or Z axis, castings showed enlargements with a mean difference of 1.177% between the palatal and the edentuious ridge planes. Results indicate that these castings were characterized by horizontal cross arch shrinkage at the level of the occlusal plane and by an increased vertical distance between the ridge plane and palatal vault. Since the casting design did not allow direct measurement, AX was calculated to provide a perpendicular comparison for BC and DE. Although this anterior posterior me~urement AX exhibited both increases and decreases,the mean change was essentially neutral. The means (Table III) were tested for significant differences from a mean change of zero using Student’s t test. 312
Fig. 6. Horizontal shrinkage pattern illustrating both increase and decrease in AX when measured distances showed only shrinkages. Frame distance AX was smaller than master cast distance AX when difference between shrinkages (in percentages) of angles DAE and distance DE was large, and greater when their differences (in percentages) were small.
With the exception of AX (p = 0.3093), all were significantly different from zero (AD, AE, BC, DE, and angle DAE, p =
titanium
Ronald Blackman, D.D.S., M.S.D.,* Nasser Barghi, D.D.S., M.A.,** Christopher Tran*** University of ‘Texas Health Science Center, Dental School, San Antonio, Texas
removable
partial
and
The difficulties encountered in casting titanium and its alloys have until recently hindered any widespread use of titanium in restorative dentistry. Now both equipment and materials are available for the routine use of titanium in the dental laboratory. This study used one of the casting systems and examined dimensional changes tha.t occur during the construction of a removable partial denture framework. Nineteen castings were measured for horizontal and vertical plane changes. Results show these castings to have cross arch contraction, vertical plane expansion, and near neutral anterioposterior change. It seems that pure titanium is within the range of dimension changes generally accepted for base metal removable partial denture alloys of the nickel-chromium variety. (J PROSTEET DENT 1991;65:309-15.)
A
lthough titanium is not new to dentistry, its application in making cast restorations introduces a new class of metals for dental treatments. Titanium casting technology has been developed and refined over many years in the aerospace industry, where casting to near net shape results in great labor and material savings. From an industrial viewpoint, titanium is attractive for its low weight-to-volume, high strength-to-weight, fatigue resistance, and corrosion resistance.’ These characteristics are desirable in dentistry, but interest is more sharply focused on biocompatibility. Titanium is hypoallergenic and possessesmany of the clinically favored properties of type III and IV dental gold alloys2 Dental interest in titanium has been restrained by the inability to overcome inherent casting problems. Now that titanium casting technology is reasonably economical and practical, its application for routine castings can be examined. Titanium casting requirements deviate considerably from those of common dental alloys. The melting point of pure titanium is 1720’ C (3646’ F) and is usually achieved with an electric arc melting method not used in dentistry. Molten titanium iis extremely reactive with other elements such as nitrogen and oxygen and with compounds such as the silica used in casting investments. When cooling from a molten state, titanium crystalizes in an alpha phase below 883’ C (1872” F). Alpha phase mechanical properties are similar to those of type III and IV dental gold alloys. Above the critical 883’ C temperature, crystalization
*AssociateProfessor,Department of Restorative Dentistry. **Professor and Head, Division of Occlusion, Department of Restorative Dentistry. ***Second year dental student. 10/I/24205
TEE JOURNAL
OF PROSTHETIC
DENTISTRY
occurs in a beta phase characterized by brittleness and increased strength. For this reason, temperature control after casting is important and must be included in a complete casting process. Titanium’s light weight presents another formidable obstacle for common centrifugal force casting methods. Its atomic weight is 47.90, making it one half as heavy as typical Ni-Cr alloys and one fourth as heavy aa high gold alloys. Therefore new and expensive casting machines are necessary. Two systems available for dental use have resolved these problems differently. One (Ohara Company, Osaka, Japan) uses centrifugal force generated by a powerful motorwound spring and an argon gas melting and casting environment. The other (Iwatani Corp., Osaka, Japan) uses a vacuum/pressure casting machine with electric arc melting, in an argon gas environment. Also, industrial progress has been made with titanium alloys. A 96% alloy, titanium, aluminum, and vanadium (Ti-6Al-4V), has been highly developed, particularly with post-casting processes for improving physical properties.3 Both this alloy and pure titanium are currently used for dental endosaeous implants4 Recent studies have examined a wide variety of titanium alloys that may prove to be more ideal for dental castings than either pure titanium or Ti-6Al-4V, however, they are not yet offered commercially for dental use.5-7The application of titanium to dentistry’s popular metal-ceramic technology has been successfully investigated* and awaits commercial development. Titanium’s high melting temperature and low specific gravity should provide exceptional sag resistance during the porcelain sintering process. Some comparisons of titanium with other removable partial denture (RPD) casting metals, derived from various sources, are given in Table I. It can be seen that pure titanium RPD castings have properties similar to long-favored type IV gold alloys. This study measured dimensional changes in large dental castings made using the equipment,
309
BLACKMAN,
Vertical Plane Reference Points
BARGHI,
AND
TRAN
Refractory Cast and Investment Mold Relations
d Vertical plane dimensions [ approx.] ocdusal to palate 16mm 6mm 1omnl
Fig. 1. Cast reference point locations for vertical plane measurements. These marks transferred from master metal die were all in the same coronal plane. Palatal plane and ridge plane were parallel to each-other. -
Horizontal Reference Points and Dimensions Horizontal plane dimensions [approx]: 4Smm AbD 5omm AlDE 41 mm AbX 39mm BbC DtoE 53mm 13mm AbBC BCbDE 2Smm
Fig. 2. Reference point locations in horizontal plane used for measurements. Approximate distances between points and calculated distance AX are indicated.
materials, and procedures of the Ohara Company. RPD master cast measurements were compared with those of titanium castings made for the same casts. Differences reflect composite dimensional changes between the RPD castings and their individual master casts. Similar studies using a popular nickel-chromium (Ni-Cr) alloy (Ticonium 100, Ticonium Company, Albany, N.Y.) provide comparisons for these titanium castings.g9lo
METHODS
AND MATERIAL
Twenty titanium, 99.5% commercially pure (CP), RPD castings were made using Ohara Company equipment with the recommended materials. Technical aspects of this system have been described in detail by Szurgot et al.lr and the procedures for this project followed their outline closely with only minor exceptions in duplicating and furnace processing temperatures. The RPD master casts were 20
310
\,
/~TgzJii~ [ minor sprues and vents deleted ]
Fig. 3. Cast and mold relations. Vertical counterclockwise spin of centrifugal casting machine placed anterior segment of casting in trailing position. Sprue curvature positioned occlusal plane perpendicular to rotating arm of casting machine.
Cast RPD Frame As Recovered E
Fig. 4. Appearance of recovered RPD framework casting. Preparation for measurements included cutting off stainless steel rods on underside of casting flush with surface, and finishing rod ends with fine abrasives.
dental stone duplicates of a master metal die stylized in the shape and size of a human maxilla (Fig. 1). The die had measurement reference marks in the forms of Xs cut in the palatal vault and edentulous ridges. Reference marks for the accusal plane consisted of five small holes (0.84 mm in diameter and 4 mm long) placed with a drill press in the duplicated stone master casts (Fig. 2). The holes were perpendicular to the occlusal plane and were located in positions representative of molars, premolars, and a central incisor. A stainless steel rod 0.76 mm in diameter and 8 mm long was placed in each hole prior to duplicating the cast with reversible hydrocolloid. The rods were transferred to the refractory cast by the duplicating material in the same alignment and position. After removal from the duplicat-
FEBRUARY
1991
VOLUME
65
NUMBER
2
CASTING
TITANIUM
REMOVABLE
PARTIAL
DENTURES
Table I. Cast titanium and RPD alloys Casting
Metal Ti, 99.5% CP
Ti, 99.5% CP Ti, 99.5% CP Co-Cr Ni-Cr Au, type IV
machine
Electric arc, centrifugal Electric arc vacuum/pressure Electric arc, centrifugal Induction, centrifugal Induction, centrifugal Gas/air torch, centrifugal
CP, Commercially pure; MOE, measure of effectiveness; ber KHN, Knoop hardness number.
MOE (GN/m2)
96 228 186 90 UTS, ultimate
UTS (MPa)
735 415 540 640 800 770
JOURNAL
OF :PROSTRETIC
DENTISTRY
% Elongation
336 383 495 690 495
18.0
7.9 7.9 1.5 1.7 6.0
Sp.gr. (gm/cm3)
4.5 4.5 4.5 8.3 7.5 15.2
Melting Temp. (“C)
Hardness
215 VHN 191 KHN 236 KHN 380 VHN 340 VHN 235 VHN
1720 1700 1700 1450 1275 950
tensile strength; YS, yield strength; Sp.gr., specific gravity; VHN, Vickers hardness num-
ing material, the refractory casts with the rods in place were oven-dried and sealed with a resin spray. All 20 casts were waxed for RPD frameworks in the same manner, beginning with 24-gauge (approximately 0.5 mm) adhesive sheet wax cut to follow the form of a template. Minor connectors were made using a double thickness (approximately 1 mm) of the same sheet wax. Frame design and major sprue configurations are illustrated in Fig. 3. A minor sprue was attached to each rest, which contained a stainless steel rod. The mold crucible was developed with a plastic form and was connected to the pattern by wax sprues (6-gauge major sprues and lo-gauge minor sprues). Waxed and sprued refractory casts were invested to form ringless molds 7.8 cm long x 8.6 cm in diameter. Fifteen 18-gauge wax per:ipheral vents were joined into groups on the underside of the refractory cast before investing. They formed three open channels through the mold to the rear surface. Wax was eliminated and molds were heat-soaked in a conventional burnout furnace. These refractory molds were then transferred to a high-temperature processing furnace for mold expansion. The temperature schedule is outlined in Table II. After heat processing, refractory molds were allowed to cool in the furnace until they could be picked up with unprotected hands and placed in the casting machine. Each casting was made with a 40 gm pure titanium ingot and a machine setting of 180 A, 38 windings, and 30 psi argon gas.Recovered castings were air abraded with sand and 25 pm aluminum oxide. Sprues and vents were removed (Fig. 4). Reference rods on the underside of rest seats were cut flush with the surface, finished with fine abrasive points, and air abraded with aluminum oxide. Of the 20 castings, one was eliminated as defective and one other had minor defects that did not interfere with its use for measurements. Nineteen RPD frameworks and their individual stone master casts were measured in both horizontal and vertical planes and were compared for dimensional changes. Casts and castings were stabilized and leveled in a vise and horizontal measurements were made with a microscope at 50 power magnification (Gaertner Model M1142, Gaertner Scientific Co.rp., Chicago, Ill). Distances AD, AE, BC,
TEE
(::a)
Table II.
Casting mold preparation Phase
Burn-out 20 to 800” c Hold temp. Processing cycle 800 to 950’ C Hold temp. 950 to 1200° c Hold temp. Cool to 40” c
Time
60 minutes 30 minutes 15 minutes 15 minutes 30 minutes 30 minutes 75 minutes
and DE (Fig. 2) were measured directly. AX, the perpendicular distance between line DE and point A, was calculated. The location of points used in these measurements are identified in Figs. 1 and 2. The stone master casts and RPD frameworks were similarly mounted and leveled for the Z axis (vertical plane) measurements. These measurements were made between the palatal vault and the plane defined by the two edentulous ridges. Vertical measurements were made directly with a digital linear dial gauge (Model EG-100, Ono Sokki, Japan) at positions designated in Figs. 1 and 5, adjacent to reference marks. All were in the same cross-sectional plane, transferred from the metal die onto the master casts and titanium frameworks. These marks were located on RPD framework elements representing a palatal strap and edentulous ridge bases. To obtain single vertical measurements, distances at points a and d, and at b and c were averaged for the calculations. Data were analyzed to determine whether and to what degree a relationship existed between the identified variables using the Pearson r and Spearman rho correlation coefficients.
RESULTS Table III shows horizontal and vertical plane measurement differences, in percentage form, for the 19 castings compared. A relatively wide spread was noticeable in the measurements from casting to casting, particularly in the
311
BLACKMAN,
Vertical Plane Frame Reference Point Positions i---, . -
Horizontal
Shrinkage
AND TRAN
Pattern Master cast size comparison
A
I
BARGAI,
I
AX increase [small diflerence between an@ DAE and DE]
Ax decrease (larae dll%r%nce belween angle DAE and DE)
Fig. 6. Illustration showing location of reference points on underside (tissue side) of center section of RPD castings. Measurements in vertical plane were obtained using points immediately medial to these marks.
distance DE (Figs. 4 and 5). Mean differences were: AD, -0656%;AE, -0+704%;BC,-0.997%;andDE,-2.564%. Results are reported in percentage form to facilitate comparisons. The calculated distance AX with a mean difference of 0.092% showed considerable dependence on the distance DE and the angle DAE, both elements of triangle DAE. When the percent shrinkage of DE was proportionately close to the percent shrinkage of the angle DAE, AX showed expansion rather than shrinkage even though measured distances all indicated shrinkage (Fig. 6). The parallel cross arch measurements BC and DE had disproportionate shrinkages, such that DE shrinkages were much greater than length differences between BC and DE would suggest. To illustrate this, lines joining the castings’ horizontal reference points would be curved and distorted representations of AD and AE would occur. In the vertical plane or Z axis, castings showed enlargements with a mean difference of 1.177% between the palatal and the edentuious ridge planes. Results indicate that these castings were characterized by horizontal cross arch shrinkage at the level of the occlusal plane and by an increased vertical distance between the ridge plane and palatal vault. Since the casting design did not allow direct measurement, AX was calculated to provide a perpendicular comparison for BC and DE. Although this anterior posterior me~urement AX exhibited both increases and decreases,the mean change was essentially neutral. The means (Table III) were tested for significant differences from a mean change of zero using Student’s t test. 312
Fig. 6. Horizontal shrinkage pattern illustrating both increase and decrease in AX when measured distances showed only shrinkages. Frame distance AX was smaller than master cast distance AX when difference between shrinkages (in percentages) of angles DAE and distance DE was large, and greater when their differences (in percentages) were small.
With the exception of AX (p = 0.3093), all were significantly different from zero (AD, AE, BC, DE, and angle DAE, p =
