Powered surgical handpiece with membrane switch

FIELD OF THE INVENTION

This invention relates generally to powered surgical handpieces and, more particularly, to a powered surgical handpiece with a membrane switch that provides an audible and tactile indication of when the switch undergoes a state transition.

BACKGROUND OF THE INVENTION

In modern surgery, one of the most important instruments available to medical personnel is the powered surgical handpiece. Typically, this handpiece includes a housing that is held by a surgeon. Inside the handpiece is some type of power-generating unit. Often, this unit is a motor. However, the power-generating unit can also be a transducer that generates sonic energy or a light-emitting device that generates thermal energy. Usually some type of accessory is attached to the head end, also known as the distal end, of the handpiece. The accessory serves as the component that transfers the energy developed by the power-generating unit to the surgical site in order to accomplish the desired surgical procedure. For example burs, drill bits and shavers are attached to handpieces that contain motors. The motor rotates these accessories in order to cause the selected removal of tissue to which they are applied. Sonic and optical waveguides are attached to the distal ends of the handpiece that, respectively, have sonic and light generating units. These waveguides direct the energy that is transmitted through them to surgical sites so that the energy causes the desired end surgical affect.

The availability of powered surgical handpieces has made it possible to perform surgical procedures more quickly and more accurately than was previously possible with the manual handpieces that they replaced. Moreover, the availability of some powered surgical handpieces has made it possible to perform some surgical procedures that, previously, were difficult, if not impossible, to perform.

When constructing a powered surgical handpiece, it is often desirable to mount switches on the surface of the housing. Often, but not always, these switches are connected to a control console to which the handpiece is connected. The depression of the switches result in the change of the characteristics of the power signal that the control console applies to the handpiece's power generating unit. The modulation of this power signal causes the power generated by the handpiece to undergo a user-requested state change. These switches make it possible for the surgeon to, with a single hand, both control the position of the handpiece and the operation of its power-generating unit. Still another advantage of providing these switches is that they eliminate the need for the surgeon to have to verbally give commands to a technician who, in turn, manipulates switches or knobs on the control console in order to cause the desired state change of the handpiece power generating unit.

Currently, many handpieces are provided with carbon contact switch assemblies. This type of assembly includes a substrate. Each switch on the substrate includes two conductive surfaces that are separate from each other by a relatively small distance of 0.100 inches or less. A rubber or silicon rubber sheet is fitted over the substrate. This sheet forms the exposed outer body of the switch assembly. For each switch on the assembly, the rubber sheet is formed to have a raised boss that forms the portion of the switch that is manually depressed. This boss subtends the complementary conductive surface of the switch with which it is integral. A conductive member is mounted to the underside surface of the rubber sheet opposite the raised boss. This conductive member may be a carbon embedded pad or a disk of metal that is highly conductive. Normally, the conductive member is spaced away from the adjacent conductive surfaces on the substrate. However, the depression of the boss causes the rubber sheet to flex inwardly. This displacement of the rubber sheet results in the conductive pad coming into physical contact with the underlying, closely spaced conductive surfaces. As a result of this contact, the conductive pad completes the circuit between the substrate conductive surfaces. The completion of this circuit thus closes the connection controlled by the switch.

A disadvantage of the above described switch assembly is that the feedback it provides upon its transition from the open state to the closed state is nominal. Consequently, it is difficult for the surgeon to know whether or not the power generating unit has actually received a set of signals requiring it to undergo the state change the surgeon wanted to occur when he/she actuated the switch. Sometimes, because the surgeon did not know whether or not the switch was actuated, he/she may repeatedly depress the switch. This action can result in the handpiece power-generating unit undergoing state changes in excess of what the surgeon wanted.

It has been suggested that it would be useful to provide powered surgical handpieces with membrane type switches. A membrane switch has a small, flexible metal dome that is located over the complementary conductive surfaces on the underlying substrate. The flexing of the dome results in the dome establishing an electrical connection between the conductive surfaces on the substrate. Also, when the dome is flexed, a distinct “click” sound is heard and a distinct tactile is obtained by the person that depressed it. Collectively, this audible and tactile feedback provides the surgeon with an indication that the switch has closed the connection it is intended to establish.

However, to date, it has proven difficult to provide a handpiece with a membrane switch assembly. This is because, in order to be reused, a surgical handpiece is typically subjected to autoclaving in order to sterilize it. In autoclaving, the handpiece is placed in a chamber in which it is exposed to saturated steam which has a temperature of approximately 270° F. and is at a pressure of approximately 30 psi. This high temperature water vapor flows into most of the unsealed portions of the handpiece. If the metal parts forming a membrane switch are exposed to this water vapor, they will, in a relatively short time period, corrode. Moreover, when the other components of a membrane switch assembly are exposed to high temperatures, even in the absence of water vapor, they are likewise prone to break down. Consequently, while there have been some efforts to provide powered surgical handpieces with membrane switch assemblies, the end products have not proven particularly satisfactory.

SUMMARY OF THE INVENTION

This invention is related to a powered surgical handpiece with a membrane-type switch assembly. The individual switches of the assembly provide tactile feedback when the switches are depressed. The switch assembly of this invention is able to withstand the rigors of the sterilization process with the handpiece with which the assembly is integral.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the claims. The above and further features of the invention may be better understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1

is a perspective view of one type of powered surgical handpiece of this invention;

FIG. 2

is an exploded view of the handpiece of

FIG. 1

in which the major sub-assemblies of the membrane switch assembly are depicted;

FIG. 3

is a cross sectional view of the housing of the handpiece of this invention;

FIG. 4

is a top plan view looking into the switch cavity of the housing;

FIG. 5

is a section view of the handpiece looking rearwardly along line

5

—

5

of

FIG. 3

;

FIG. 6

is a perspective view of the switch plate of the membrane switch assembly;

FIGS. 7A

,

7

B,

7

C,

7

D,

7

E,

7

F and

7

G are plan views of the sheets of material that form the switch plate;

FIG. 8

is an exploded view depicting the relationship of the sheets of material that form the switch plate;

FIG. 9

is a schematic view of the circuit formed on the circuit layer of the switch plate;

FIG. 10

is perspective view of the top of the keypad of the switch assembly;

FIG. 11

a perspective view of the bottom of the keypad;

FIG. 12A

is a top plan view of the bezel plate of the switch assembly;

FIG. 12B

is a cross-sectional view of the bezel plate taken along lines

12

B—

12

B of

FIG. 12A

; and

FIG. 13

is a cross-sectional view of a seal used to establish a hermetic barrier around the switch assembly.

DETAILED DESCRIPTION

FIGS. 1 and 2

depict a powered surgical handpiece

20

of this invention that includes a membrane switch assembly

22

. The particular handpiece

20

illustrated is referred to as a “shaver” and is used to perform endoscopic surgery. An understanding of endoscopic surgery can be obtained from the Applicant's Assignee's U.S. Pat. No. 5,850,496, entitled ENDOSCOPE WITH INTEGRATED, SELF-REGULATING LIGHT SOURCE, issued Dec. 15, 1998 which. is incorporated herein by reference. The handpiece

20

includes an elongated housing

24

that forms the body of the handpiece. An electric motor

26

is disposed in the housing

24

. More particularly, the motor

26

is seated in a cylindrically shaped motor cavity

27

that extends forward from the rear, proximal end of the housing

24

. Energization signals are applied to the motor

26

over a cable

28

attached to the proximal, rear end of the housing. The energizaton signals are generated by a control console (not illustrated) to which the opposed end of the cable

28

is connected. A coupling assembly

30

is mounted to the front, distal end of the housing

24

. More specifically, coupling assembly

30

is mounted in a portion of the motor cavity

27

that extends rearwardly from the distal end of the housing

24

. Coupling assembly

30

is used to releasably hold a cutting accessory (not illustrated) to the handpiece

20

. When the cutting accessory is coupled to the handpiece

20

, the accessory is fitted to an output drive shaft

32

that is connected to the rotor (not illustrated) integral with the motor

26

. Actuation of the motor

26

results in rotational movement of the cutting accessory. The head end of the cutting accessory is applied to a surgical site in order to accomplish a desired surgical procedure.

The depicted housing

24

, as seen in

FIGS. 2 and 3

, is also provided with a conduit through which suction may be drawn. It should be understood that the cutting accessories with which handpiece

20

are used typically have tubular bodies that are open at their proximal ends. The handpiece housing

24

is formed to have a pair of bores

34

and

36

. Bore

34

extends diagonally rearwardly away from the motor cavity

27

at a location immediately rearward of the distal end of the housing. Bore

36

, which runs parallel to the motor cavity

27

, extends to the proximal end of the housing

24

. A suction fitting

37

extends out of bore

36

beyond the proximal end of housing

24

. When the handpiece

20

is in use a suction pump is connected to fitting

37

over a suction tube (pump and tube not illustrated). The suction drawn by the pump causes a suction flow from the front end of the cutting accessory, through the cutting accessory, the bores

34

and

36

and the suction tube to a receptacle integral with the pump. This suction flow is used to remove fluid and debris from the surgical site.

A valve

38

fitted to the housing

24

regulates the suction flow. Valve

38

is located in a valve chamber

40

formed in the housing

24

between bores

34

and

36

. The position of the valve

38

in chamber

40

controls the magnitude of the suction flow.

The membrane switch assembly

22

is seated in a switch cavity

42

formed in the housing

24

. Specifically, the switch cavity

42

, now described by reference to

FIGS. 4 and 5

, is an elongated space that is formed immediately rearwardly of the distal end of the housing

24

. Specifically, housing

24

is formed so as to have two longitudinally extending, spaced apart outer steps

44

. The outer steps are surfaces of the housing

24

that are recessed relative to the outer surface of the housing. As will be described hereinafter, the outer steps

44

are the surfaces of the housing

24

against which a bezel plate

46

(FIG.

2

), which forms the outer body of the switch assembly

22

, seats. Housing

24

is further formed so as to have an inner step

48

that extends between and is located adjacent to the outer steps

44

. Inner step

48

is inwardly recessed relative to the outer steps

44

. The inner step

48

is generally in the form of a rectangular frame. The inner perimeter of the inner step

48

defines the outer perimeter of switch cavity

42

. The housing

24

is further formed so as to have a groove

50

that extends inwardly from the top surface of the inner step

48

. Groove

50

extends circumferentially around the whole of the surface of inner step

48

. It should further be understood that the housing

24

is formed so that the outer steps

44

and inner step

48

have an arcuate cross-sectional profile that approximates the curvature of the outer surface of the housing

24

from which the steps

44

and

48

are recessed.

Handpiece housing

24

is further formed so as to have two spaced-apart, longitudinally extending raised surfaces

52

that extend upwardly into the base of switch cavity

42

. Collectively, the raised surfaces

52

define an elongated rectangular slot

54

that extends longitudinally and axially along the base of the switch cavity

42

. The housing

24

is formed so as to have a front wall

55

that closes the front end of the switch cavity

42

. Housing

24

has a rear wall

56

that defines the proximal end of cavity

42

. Rear wall

56

is shaped to have an opening

60

that is aligned with and extend upwardly a small distance from the proximal end of slot

54

. Opening

60

provides communication between motor cavity

27

and switch cavity

42

.

Membrane switch assembly

22

includes a switch plate

64

now described by reference to

FIGS. 6

,

7

A-

7

G and

8

.

The switch plate

64

supports the conductive components of the switch assembly

22

. A backing plate

66

(

FIG. 7A

) serves as the support substrate for the remaining components of the switch plate

64

. The backing plate

66

is formed of metal such as aluminum or other rigid material. In some versions of the invention, backing plate

66

is approximately 0.040 inches thick. The backing plate

66

is shaped to have a generally rectangular profile that facilitates the slip fitting of the plate in switch cavity

42

. The corners of the backing plate

66

are, however, beveled to facilitate the insertion and removal of the switch plate

64

in the switch cavity. Generally, the layer components of the switch assembly

64

disposed on the backing plate

66

have a profile identical to that of the backing plate. The backing plate

66

is further formed to define an opening

68

that is slightly offset from the center of the plate. The reason for providing opening

68

is discussed below.

Immediately above the backing plate

66

, switch plate

64

has a cover layer

70

(FIG.

7

B). Cover layer

70

is formed of a dielectric material such as Kapton and in some versions of the invention is 0.002 inches thick. The cover layer

70

serves as a protective and insulating layer for the circuitry and components that are located above it. An insulating lamination layer

72

(

FIG. 7C

) is located above cover layer

70

. Lamination layer

72

is formed from the same material of, and has the same thickness as, cover layer

70

. The lamination layer covers, protects and insulates the next-described layer, the circuit layer

74

. The lamination layer

72

, like the layer it protects, is formed to have a stem section

76

that extends from one end of the main body of the layer. Lamination layer is further formed to define a rectangular center opening

78

. Opening

78

of lamination layer

72

is positioned to be in registration with opening

68

of backing plate

66

.

The circuit layer, layer

74

(

FIG. 7D

) is the layer of switch plate

64

.that physically carries the conductive components of the switch plate. Circuit layer

74

is a Kapton layer to which conductive traces have been selectively applied. The circuit formed on layer

74

is illustrated by FIG.

9

. Specifically, there is a master bus

80

through which a reference signal flows. The reference signal is applied to bus

80

through a trace

79

and a resistor

81

. In the illustrated version of this invention, this reference signal is a 5 VDC signal and resistor

81

is a 1 K Ohm resistor. The reference signal from bus

80

is applied to three generally rectangular conductive frames

86

formed on the outer, upper surface of circuit layer

74

. The top of the conductive layer is further formed so that a conductive pad

87

is located in the center of each frame

86

. It should be understood that each frame

86

and complementary pad

87

are physically spaced apart from each other. Also, there is a small opening in each frame

86

to provide a space for a trace connected to the associated pad

87

.

The conductive pads

87

are electrically connected to traces (not illustrated) located on the underside of circuit layer

74

. More specifically, each pad

87

is connected to a separate resistor

88

,

89

and

90

that is mounted to the underside of layer

74

. Resistors

88

,

89

and

90

are series connected together. The resistors

88

,

89

and

90

have resistances of 365, 240 and 180 Ohms, respectively. Resistor

90

is connected to a trace

94

that leads to a ground plane. Trace

94

is located on the underside of the circuit layer

74

. It should also be understood that resistors

81

,

88

,

89

and

90

are mounted to the underside of circuit layer

74

.More particularly, the resistors are located on a surface of the underside of circuit layer

74

that is subtended by backing plate opening

68

and lamination layer opening

78

.

Circuit layer

74

, it will be further observed, is formed with a stem section

96

. While not-seen in

FIG. 7D

, it should be understood that traces

79

and

94

extend outwardly on the stem section

96

. There is also a third trace, trace

92

. Trace

92

extends outwardly from the master bus

80

.

An insulating and protective lamination layer

102

(

FIG. 7E

) is located over the circuit layer. Lamination layer

102

is formed from the same material as, and has the same thickness as, lamination layer

72

. Lamination layer

102

is shaped to define three spaced apart rectangular openings

104

. Each opening

104

is positioned to subtend a separate one of the conductive frames

86

. Lamination layer

102

is also formed to define two channels

106

and

108

. The channels

106

and

108

are narrower in width than openings

104

. Channel

106

connects two adjacent openings

104

. Channel

108

connects the other two openings

104

. Lamination layer

102

has a. stem section

110

that extends over the top of cover layer stem section

96

.

A cage layer

112

(

FIG. 7F

) is located above lamination layer

102

. Cage layer

112

is formed from the same material as lamination layer

102

. The material forming cage layer

112

has a thickness between 0.005 and 0.010 inches. The cage layer

112

has a rectangular shape that approximates that of the switch plate

64

or cover layer

70

. Cage layer

112

is further shaped to define three openings

114

and two channels

116

and

118

. Each opening

114

is positioned to be in registration over and has the same size as a separate one of the lamination layer openings

104

. Channels

116

and

118

are positioned to be in registration over and subtend the same area as lamination layer channels

106

and

108

, respectively.

Switch plate

64

is also provided with three domes

120

. Each dome is formed of flexible metal such as stainless steel that is approximately 0.004 inches thick. The conductive metal forming the dome

120

has a click force of between 300 and 400 grams. Each dome

120

has a body

122

with a curved profile that approximates the curvature of a section of a sphere. Four legs

124

, that are equangularly spaced, extend downwardly from the body

122

. Each dome is seated over a separate one of the Ha conductive frames

86

. The feet

124

of a dome physically abut the conductive material forming the associated frame. The dome body

122

normally extends over the conductive pad

87

centered in the frame

86

. It should also be understood that each pair of lamination layer and cage layer openings

104

and

114

, respectively, collectively define a space in which an associated one of the domes

120

is seated.

A cover layer

126

(

FIG. 7G

) extends over the cage layer

112

and is the topmost layer of the switch plate

64

. Cover layer

126

is formed from the same material as and has the same thickness as the lamination layer

102

. Cover layer

126

has the same basic shape as the first described cover layer

70

. Cover layer

126

does, however, have a relatively short stem

127

that extends a short distance over lamination layer stem section

110

. It will be observed that cover layer

126

extends over the tops of domes

120

so as to hold the domes in place. Cover layer

126

is formed with a small vent hole

128

. Vent hole

128

is centered over cage layer channel

118

. Vent hole

128

has a diameter of approximately 0.05 inches.

As represented by dashed lines

130

in

FIG. 8

, thermal sensitive adhesive is used to bond: cover layer

70

to backing plate

66

; lamination layer

72

to cover layer

70

; circuit layer

74

to lamination layer

72

; lamination layer

102

to circuit layer

74

; and cage layer

112

to lamination layer

102

. However, a pressure sensitive adhesive, represented by dotted line

132

is used to bond cover layer

126

to cage layer

112

. One suitable pressure sensitive adhesive that can be used to assemble handpiece

20

is available from the

3

M Company of Minneapolis, Minn. as Transfer Tape Product No. 9461P.

As seen by reference to

FIG. 7F

, in which the pressure sensitive adhesive is shown as a stippled surface

134

, the adhesive is applied to the surface of cage layer

112

so that it is spaced away from the edges of the cage layer

112

that define openings

114

. For example, in some versions of the invention, the pressure sensitive adhesive is applied to the cage layer

112

in such a manner that it is spaced between 0.020 and 0.025 inches away from the edges of the layer

112

that define openings

114

.

A keypad

138

, now described by reference to

FIGS. 10 and 11

, covers the switch plate

64

. The keypad

138

is formed from a single piece of flexible silicone rubber. The keypad

138

has a solid body

140

. In cross section, keypad body

140

has a flat bottom surface and an opposed top surface that has an arcuate profile. Longitudinally, the keypad body

140

is shaped so that the body is able to snugly fit in switch cavity

42

. Widthwise, the body

140

is shaped so as to not extend beyond the lateral perimeter of switch cavity

42

. The keypad is further formed to have three solid, cylindrical heads that project upwardly from the top surface of the body

140

. The heads are the individual buttons

142

of the switch assembly

22

that are selectively depressed. When the handpiece

20

is assembled, each button is in registration over a separate one of the domes

120

. It will be further observed that the keypad body

140

is shaped so that the annular sections of material that surround buttons

142

are of reduced thickness so as to define annular recesses

143

around the buttons

142

. Extending upwardly from the surfaces of the keypad body

140

that define recesses

143

, the body is further formed to have a number of cylindrical bosses

144

. The top surfaces of bosses

144

are generally flush with the top surfaces of the rest of the keypad body

140

.

In the illustrated version of the invention, the top surfaces of the buttons

142

are formed to define identification numbers “1”, “2” and “3”. Also, the top surfaces of the buttons

142

are formed to have distinct profiles. The distal button

142

is formed with small, raised bosses

153

that surround its top surface and the identification number “1” is raised relative to the top surface. The middle button

142

has a smooth top surface and the identification number “2” is formed as a recess in the button. The proximal button

142

is formed with a raised lip around its outer perimeter and the identification number “3” is raised relative to its top surface. The individual buttons

142

are formed differently in order to provide a person using the handpiece

20

with different tactile impressions for each button. This feature of the invention is intended to minimize the extent to which a person has to look at a button in order to determine which button she is pressing.

The underside of the keypad is formed so that the body has three circular indentations

145

. Each indentation subtends the area subtended by a corresponding one of the buttons

142

. The circular indentations

145

are connected by channels

146

. The keypad

138

is further formed so that centered within each indentation

145

is an even deeper counterbore

147

. The counterbores

147

are concentric with the indentions with which the counterbores are associated. A small cylindrical plunger

141

extends outwardly from the base of each counterbore

147

. The top of each plunger

141

is flush with the base surface of the associated indentation

145

.

Keypad

138

is further shaped to have a lip

148

that extends outwardly beyond the perimeter of body

140

. Lip

148

is shaped to have an arcuate shape. The arc inscribed by the keypad lip

148

is the arc inscribed by the space immediately above inner step

48

of the housing

24

. Thus, lip

148

is dimensioned to seat upon the exposed top surface of inner step

48

. Lip

148

is dimensioned to have a thickness that is slightly greater than the depth of the space in which it is seated. For example, in one version of the invention inner step

48

is recessed 0.033 inches relative to the adjacent outer steps

44

. The keypad lip

148

, which seats in this recessed space, has a thickness of 0.036 inches.

Bezel plate

46

, illustrated in

FIGS. 12A and 12B

, seats over switch plate

64

and keypad

138

. The bezel plate

46

is formed from hardened stainless steel. The bezel plate

46

is shaped to have a curved profile and is further dimensioned to seat immediately on top of the housing outer steps

44

. Three openings

154

extend through the bezel plate

46

. The openings are located along the longitudinal center axis of the bezel plate

46

. Each opening

154

accommodates a separate one of the buttons

142

integral with keypad

138

. Screws

156

(

FIG. 2

) secure the bezel plate

46

to the housing

24

. The screws

156

extend through screw holes

158

formed in the side of the bezel plate

156

and fit into threaded bores

160

that extend inwardly from the housing inner steps

44

.

Returning to

FIG. 2

, it can be seen that another component of the handpiece

20

of this invention is an elongated flex circuit

164

. Flex circuit

164

extends forward from the distal end of the motor

26

, through housing opening

60

into switch cavity

42

. Flex circuit

164

carries conductors (not illustrated) that provide an electrical path between the switch plate

64

and a plug

166

mounted to the distal end of the motor

26

. The plug

166

is the component to which cable

28

is connected. Thus, the flex circuit

164

serves as one of the conductive members through which signals are exchanged between the switch assembly

22

and the control console.

The flex circuit

164

extends through a seal

168

now described by reference to

FIGS. 2 and 13

. Seal

168

is formed from silicone rubber. The seal

168

has a body

169

that is formed to have arcuately shaped top and bottom surfaces. More particularly, the seal body

169

is shaped so that the bottom surface has an arcuate profile that approximates the curvature of a forward section

170

of the housing in which the motor and associated reduction gears are seated. The top surface of the seal

168

is shaped to have a curvature that approximates that of a section

172

of the motor cavity

27

(

FIG. 3

) in which the seal is seated. Seal

168

is also shaped to have a tail section

174

that extends rearwardly from the proximal end of the seal body

169

.

The seal

168

is shaped to have an elongated slot

176

that extends longitudinally through the body of the seal. More particularly, the seal

168

is formed so that its proximal end and the tail section

174

define a rear opening

178

into the slot that is relatively wide in the lateral direction. Forward from opening

178

, slot

176

narrows in height. Adjacent the front of the seal, the seal is shaped so that slot

176

has dimensions that approximate that of the flex circuit

164

.

Housing

24

is formed so that the motor cavity has a section

172

shaped to accommodate seal

168

. Section

172

is located immediately rearwardly of housing rear wall

56

.

When the handpiece

20

of this invention is assembled, seal

168

is fitted over flex circuit

164

. The seal is positioned so that the tail section

174

seats against a shoulder

180

of the motor housing that defines the proximal end of the forward section

170

of the motor housing. Collectively, the housing

24

, the seal

168

and the motor housing are shaped so that when the motor is completely positioned in the housing the longitudinal separation between the rear face of housing wall

64

and motor housing shoulder

180

is less than the length of the seal body

169

. Consequently, when the motor

26

, the flex circuit and the seal

168

sub assembly are fitted in housing

24

, the seal body

169

is compressed between the housing rear wall

56

and the motor housing shoulder

180

. This compression urges the front end of the seal body

169

rot against the adjacent surface of rear wall

56

. The compression also urges the portions of the seal body

169

around the proximal portion of slot

176

against the flex circuit

164

. As a consequence of the seal

168

abutting these surfaces, the seal forms a hermetic barrier around the flex circuit

164

and the opening

60

in the housing rear wall

56

.

Handpiece

20

of this invention is assembled by positioning switch plate

64

in housing switch cavity

42

. The switch plate

64

is oriented so that the stem sections

76

,

96

,

110

and

127

extend in the forward direction. More specifically, the stem sections

76

,

96

and

110

are folded around the front end of plate

64

so as to be located underneath the backing plate

64

. Conductive vias on circuit layer stem section

96

are bonded to complementary conductive pads on flex circuit

164

(vias and pads not illustrated). When the switch-plate

64

is fitted into switch cavity

42

, the backing plate

66

seats on raised surfaces

52

; the distal end of the flex circuit

164

and stem sections

76

,

96

and

110

seat in slot

54

.

After the switch plate

64

is connected to the flex circuit and seated in the switch cavity

42

, keypad

138

is fitted in place over the switch plate., Keypad body

140

seats in the top of the switch cavity

42

. The keypad lip

148

extends over and abuts housing inner step

48

.

Bezel plate

46

is then fitted over switch plate

64

and keypad

138

. More particularly, the handpiece housing

24

and bezel plate

46

are collectively dimensioned so that, eventually, as a result of screws

156

pushing the bezel plate downwardly, the inner surface of the bezel plate abuts the exposed surfaces of housing outer steps

44

. Also, as a result of the pressure imposed on the key pad lip

148

, a small rib like section of the lip is forced into groove

50

.

Collectively, a single switch of assembly

22

consists of: a conductive frame

86

and its complementary conductive pad

87

; the dome

120

that extends over the conductive frame; and the keypad button

142

and plunger

141

located over the dome. Resistor

82

, in combination with resistors

88

-

90

, form a variable resistance voltage divider. The closing of the switches determines which of the resistors

88

,

89

and/or

90

are part of the voltage divider. The voltage between trace

92

and trace

94

serves as a measure of the resistance across the voltage divider. Therefore, the voltage across traces

92

and

94

serve as an indication of which of the switches is in the closed state.

When a surgeon wants to actuate one of the switches on switch assembly

22

, he/she depresses the appropriate button

142

. The button

142

will flex inwardly. The plunger

141

associated with the button

142

strikes the underlying dome