Technology in Action
resulting in a gradual increase in the average age. Te most rapid growth, about 3.5% a year, is in those 65 years and above. Coincidentally, the average age for knee surgery is 65. Te other major trend contributing to a surge in orthopedic implants is the growing number of persons who are overweight or obese. Approximately 1.57 billion of the world’s 7.2 billion people are overweight, and 0.53 billion are classed as clinically obese (BMI > 30%). Excess weight increases the likelihood of the develop- ment of osteoarthritis, a major reason for joint replacement. Typically, a total knee replacement
consists of three subcomponents: the femoral component, which replaces the rounded bottom end of the femur bone; the tibial tray, which replaces the top end of the tibia bone; and the tibial or bearing insert, which fits between and cushions the other two parts. The bearing insert usually is produced from UHMWPE (Ultra High Molecular Weight Polyethylene, an engineering polymer), whereas the femoral component and tibial tray are in most cases produced from cobalt chrome (Co-Cr) alloy or in some cases a titanium alloy. These alloys are strong and hard, biocompatible materials with high stiffness (Youngs modulus) and abrasiveness when be- ing machined.
Machining the Femoral Component Machining techniques for femoral
components include both grinding and milling. Te challenges are to achieve a burr-free profile with superior surface finish that minimizes the need for manual polishing, and at the same time maximizes productivity and tool life. For these tough milling operations, Seco has developed specially-de- signed tapered ballnose cutters and modified Jabro JHP770 high-performance cutters that feature differential flute spacing to minimize vibration during operation. Among the machining methods employed are corner plunging, periph- ery machining, box roughing and finishing, cam finishing and box-blend machining. Te femoral component has rounded contours that mimic
the condyle bone formation at the end of the femur. Te shape has traditionally been produced via grinding, but that operation can generate high temperatures that may distort the part. Seco has developed tools and performed tests to replace the grinding
70 Medical Manufacturing 2014 A Jabro Premier Finish cutter.
process with milling. A large medical OEM performed trials with the tools, finishing a cast Co-Cr femoral component with a copy milling strategy that employed a special solid-carbide Jabro ballend mill. Te result was cycle-time reductions of up to 11 minutes per part, representing 50% less time compared to the grinding method used previously. Tool life exceeded 12 hours, enabling one cutter to machine more than 80 parts. Excellent control of radial depth of cut on a five-axis milling machine contributed to the extended tool life. In four-axis applications without such control, tool life reached 6–8 hours. Te change from grinding to milling also eliminated the possibility of scrap parts due to distortion.
Machining the Tibial Tray and Bearing Insert Machining the Co-Cr tibial tray
also presents challenges in terms of surface finish and productivity requirements. In addition, the part has right-angle locking details that must be produced burr-free. Machining the part typically can take up to seven separate machining operations. To achieve a superior finish on the
base where the tibial insert is seated, a new Seco multiflute cutter with special wiper geometry was applied. Te tool has produced Ra
values of below 0.1
µm. In machining operation 6, Seco implemented a combined wall finish/ chamfer cutter. Te combination of finish and chamfer tools provides a controlled way of mechanical edge pro- filing (MEP) and prevents secondary burrs while eliminating manual rework and reducing tool costs.
Bearing inserts for replacement knees typically are made
of UHMWPE. This material is relatively soft and therefore generates low cutting forces, but surface roughness require- ments of 0.10 µm Ra demand that it be machined with sharp, top-quality finishing tools. Under its Jabro brand, Seco developed the Premier Finish solid end mill designed to meet the specific requirements of a leading global medi- cal OEM.
Condyle Contour Machining Difficulties Te condyle shape of both the femoral component and the
bearing insert can be difficult to machine. Previous to the development of the Premier Finish end mills, condyle surfaces were machined using polished HSS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216